diff --git a/app/controllers/domains_controller.rb b/app/controllers/domains_controller.rb
index 146df1d..229d27e 100644
--- a/app/controllers/domains_controller.rb
+++ b/app/controllers/domains_controller.rb
@@ -98,9 +98,48 @@ class DomainsController < ApplicationController
     redirect_to [:verify, organization, @server, @domain], alert: "You can't set up DNS for this domain until it has been verified."
   end
 
+  def regenerate_dkim
+    if @domain.pending_dkim_key?
+      redirect_to_with_json [:setup, organization, @server, @domain],
+                            alert: "A DKIM key change is already in progress for #{@domain.name}. Publish the new " \
+                                   "record shown below, or cancel the change, before generating another key."
+      return
+    end
+
+    was_verified = @domain.dkim_status == "OK"
+    @domain.regenerate_dkim_key!
+    if was_verified
+      redirect_to_with_json [:setup, organization, @server, @domain],
+                            notice: "A new DKIM key has been generated for #{@domain.name}. Add the new DNS record below — " \
+                                    "your existing key will remain active until the new record has been verified. Keep the " \
+                                    "old DNS record published afterward while messages signed with it may still be queued, " \
+                                    "held, or available for redelivery."
+    else
+      redirect_to_with_json [:setup, organization, @server, @domain], notice: "A new DKIM key has been generated for #{@domain.name}. Update your DKIM DNS record with the new value below."
+    end
+  end
+
+  def cancel_dkim_regeneration
+    @domain.cancel_pending_dkim_key!
+    redirect_to_with_json [:setup, organization, @server, @domain], notice: "The pending DKIM key for #{@domain.name} has been discarded. Your existing key remains active."
+  end
+
   def check
+    had_pending_dkim_key = @domain.pending_dkim_key?
+    previous_dkim_record_name = @domain.dkim_record_name
     if @domain.check_dns(:manual)
-      redirect_to_with_json [organization, @server, :domains], notice: "Your DNS records for #{@domain.name} look good!"
+      if had_pending_dkim_key && !@domain.pending_dkim_key?
+        redirect_to_with_json [:setup, organization, @server, @domain],
+                              notice: "Your DNS records for #{@domain.name} look good! Your new DKIM key is now active. " \
+                                      "Keep the old record at #{previous_dkim_record_name} published while messages " \
+                                      "signed with the old key may still be queued, held, or available for redelivery."
+      elsif @domain.pending_dkim_key?
+        redirect_to_with_json [:setup, organization, @server, @domain],
+                              alert: "We couldn't verify the record for your new DKIM key yet. Your existing key remains " \
+                                     "active. Check below for the expected record details."
+      else
+        redirect_to_with_json [organization, @server, :domains], notice: "Your DNS records for #{@domain.name} look good!"
+      end
     else
       redirect_to_with_json [:setup, organization, @server, @domain], alert: "There seems to be something wrong with your DNS records. Check below for information."
     end
diff --git a/app/models/concerns/has_dns_checks.rb b/app/models/concerns/has_dns_checks.rb
index 789aeff..97e0406 100644
--- a/app/models/concerns/has_dns_checks.rb
+++ b/app/models/concerns/has_dns_checks.rb
@@ -72,6 +72,17 @@ module HasDNSChecks
   #
 
   def check_dkim_record
+    # If a new DKIM key is waiting to be activated, promote it to be the active
+    # key as soon as its DNS record is in place. The matching DNS response has
+    # already verified the key, so avoid a second lookup which could return a
+    # different answer while DNS changes are propagating.
+    if pending_dkim_key? && pending_dkim_record_live?
+      activate_pending_dkim_key
+      self.dkim_status = "OK"
+      self.dkim_error = nil
+      return true
+    end
+
     domain = "#{dkim_record_name}.#{name}"
     records = resolver.txt(domain)
     if records.empty?
@@ -98,6 +109,16 @@ module HasDNSChecks
     save!
   end
 
+  # Returns true if the DNS record for the pending DKIM key has been published
+  # correctly.
+  def pending_dkim_record_live?
+    records = resolver.txt("#{pending_dkim_record_name}.#{name}")
+    return false unless records.size == 1
+
+    sanitised_record = records.first.strip.ends_with?(";") ? records.first.strip : "#{records.first.strip};"
+    sanitised_record == pending_dkim_record
+  end
+
   #
   # MX
   #
diff --git a/app/models/domain.rb b/app/models/domain.rb
index fcd4eb3..cb63929 100644
--- a/app/models/domain.rb
+++ b/app/models/domain.rb
@@ -4,31 +4,33 @@
 #
 # Table name: domains
 #
-#  id                     :integer          not null, primary key
-#  server_id              :integer
-#  uuid                   :string(255)
-#  name                   :string(255)
-#  verification_token     :string(255)
-#  verification_method    :string(255)
-#  verified_at            :datetime
-#  dkim_private_key       :text(65535)
-#  created_at             :datetime
-#  updated_at             :datetime
-#  dns_checked_at         :datetime
-#  spf_status             :string(255)
-#  spf_error              :string(255)
-#  dkim_status            :string(255)
-#  dkim_error             :string(255)
-#  mx_status              :string(255)
-#  mx_error               :string(255)
-#  return_path_status     :string(255)
-#  return_path_error      :string(255)
-#  outgoing               :boolean          default(TRUE)
-#  incoming               :boolean          default(TRUE)
-#  owner_type             :string(255)
-#  owner_id               :integer
-#  dkim_identifier_string :string(255)
-#  use_for_any            :boolean
+#  id                             :integer          not null, primary key
+#  dkim_error                     :string(255)
+#  dkim_identifier_string         :string(255)
+#  dkim_private_key               :text(65535)
+#  dkim_status                    :string(255)
+#  dns_checked_at                 :datetime
+#  incoming                       :boolean          default(TRUE)
+#  mx_error                       :string(255)
+#  mx_status                      :string(255)
+#  name                           :string(255)
+#  outgoing                       :boolean          default(TRUE)
+#  owner_type                     :string(255)
+#  pending_dkim_identifier_string :string(255)
+#  pending_dkim_private_key       :text(65535)
+#  return_path_error              :string(255)
+#  return_path_status             :string(255)
+#  spf_error                      :string(255)
+#  spf_status                     :string(255)
+#  use_for_any                    :boolean
+#  uuid                           :string(255)
+#  verification_method            :string(255)
+#  verification_token             :string(255)
+#  verified_at                    :datetime
+#  created_at                     :datetime
+#  updated_at                     :datetime
+#  owner_id                       :integer
+#  server_id                      :integer
 #
 # Indexes
 #
@@ -82,7 +84,7 @@ class Domain < ApplicationRecord
   end
 
   def generate_dkim_key
-    self.dkim_private_key = OpenSSL::PKey::RSA.new(1024).to_s
+    self.dkim_private_key = OpenSSL::PKey::RSA.new(Postal::Config.dns.dkim_key_size).to_s
   end
 
   def dkim_key
@@ -91,6 +93,57 @@ class Domain < ApplicationRecord
     @dkim_key ||= OpenSSL::PKey::RSA.new(dkim_private_key)
   end
 
+  def pending_dkim_key
+    return nil unless pending_dkim_private_key
+
+    @pending_dkim_key ||= OpenSSL::PKey::RSA.new(pending_dkim_private_key)
+  end
+
+  def pending_dkim_key?
+    pending_dkim_private_key.present?
+  end
+
+  # Generate a new DKIM key for this domain. If the current key is verified and
+  # in use for signing, the new key is stored as a pending key under a new
+  # identifier so that signing continues with the current key until the new
+  # key's DNS record has been published and verified. Otherwise, the key is
+  # replaced immediately and any in-flight key change is abandoned, since the
+  # new key supersedes it.
+  def regenerate_dkim_key!
+    if dkim_status == "OK"
+      self.pending_dkim_private_key = OpenSSL::PKey::RSA.new(Postal::Config.dns.dkim_key_size).to_s
+      self.pending_dkim_identifier_string = generate_unique_dkim_identifier_string
+    else
+      generate_dkim_key
+      self.dkim_status = nil
+      self.dkim_error = nil
+      self.pending_dkim_private_key = nil
+      self.pending_dkim_identifier_string = nil
+      @dkim_key = nil
+    end
+    @pending_dkim_key = nil
+    save!
+  end
+
+  # Promote the pending DKIM key to be the active key. Does not save the record.
+  def activate_pending_dkim_key
+    return unless pending_dkim_key?
+
+    self.dkim_private_key = pending_dkim_private_key
+    self.dkim_identifier_string = pending_dkim_identifier_string
+    self.pending_dkim_private_key = nil
+    self.pending_dkim_identifier_string = nil
+    @dkim_key = nil
+    @pending_dkim_key = nil
+  end
+
+  def cancel_pending_dkim_key!
+    self.pending_dkim_private_key = nil
+    self.pending_dkim_identifier_string = nil
+    @pending_dkim_key = nil
+    save!
+  end
+
   def to_param
     uuid
   end
@@ -108,23 +161,27 @@ class Domain < ApplicationRecord
   end
 
   def dkim_record
-    return if dkim_key.nil?
-
-    public_key = dkim_key.public_key.to_s.gsub(/-+[A-Z ]+-+\n/, "").gsub(/\n/, "")
-    "v=DKIM1; t=s; h=sha256; p=#{public_key};"
+    build_dkim_record(dkim_key)
   end
 
   def dkim_identifier
-    return nil unless dkim_identifier_string
-
-    Postal::Config.dns.dkim_identifier + "-#{dkim_identifier_string}"
+    build_dkim_identifier(dkim_identifier_string)
   end
 
   def dkim_record_name
-    identifier = dkim_identifier
-    return if identifier.nil?
+    build_dkim_record_name(dkim_identifier)
+  end
 
-    "#{identifier}._domainkey"
+  def pending_dkim_record
+    build_dkim_record(pending_dkim_key)
+  end
+
+  def pending_dkim_identifier
+    build_dkim_identifier(pending_dkim_identifier_string)
+  end
+
+  def pending_dkim_record_name
+    build_dkim_record_name(pending_dkim_identifier)
   end
 
   def return_path_domain
@@ -160,6 +217,36 @@ class Domain < ApplicationRecord
 
   private
 
+  def build_dkim_record(key)
+    return if key.nil?
+
+    public_key = key.public_key.to_s.gsub(/-+[A-Z ]+-+\n/, "").gsub(/\n/, "")
+    "v=DKIM1; t=s; h=sha256; p=#{public_key};"
+  end
+
+  def build_dkim_identifier(identifier_string)
+    return nil unless identifier_string
+
+    Postal::Config.dns.dkim_identifier + "-#{identifier_string}"
+  end
+
+  def build_dkim_record_name(identifier)
+    return if identifier.nil?
+
+    "#{identifier}._domainkey"
+  end
+
+  # Generates a random identifier string in the same format as the
+  # `random_string :dkim_identifier_string` declaration, unique across both the
+  # active and pending identifier columns.
+  def generate_unique_dkim_identifier_string
+    loop do
+      string = Nifty::Utils::RandomString.generate(length: 6, upper_letters_only: true)
+      scope = self.class.where(dkim_identifier_string: string).or(self.class.where(pending_dkim_identifier_string: string))
+      return string unless scope.exists?
+    end
+  end
+
   def update_verification_token_on_method_change
     return unless verification_method_changed?
 
diff --git a/app/views/domains/index.html.haml b/app/views/domains/index.html.haml
index 2238e01..55c66ec 100644
--- a/app/views/domains/index.html.haml
+++ b/app/views/domains/index.html.haml
@@ -34,6 +34,8 @@
               = link_to domain.name, [:setup, organization, @server, domain]
               - if domain.use_for_any?
                 %span.label.label--blue Any
+              - if domain.pending_dkim_key?
+                = link_to "New DKIM key pending", [:setup, organization, @server, domain], :class => "label label--orange", :title => "A new DKIM key is waiting for its DNS record to be published."
             %ul.domainList__checks
               - if domain.spf_status == 'OK'
                 %li.domainList__check.domainList__check--ok SPF
diff --git a/app/views/domains/setup.html.haml b/app/views/domains/setup.html.haml
index 34d9e5a..4c53198 100644
--- a/app/views/domains/setup.html.haml
+++ b/app/views/domains/setup.html.haml
@@ -47,7 +47,11 @@
   %pre.codeBlock.u-margin= @domain.spf_record
 
   %h3.pageContent__subTitle DKIM Record
-  - if @domain.dkim_status == 'OK'
+  - if @domain.pending_dkim_key?
+    %p.pageContent__text.u-orange.u-bold
+      %span.label.label--orange Key change in progress
+      You've generated a new DKIM key which isn't active yet. Both records are shown below.
+  - elsif @domain.dkim_status == 'OK'
     %p.pageContent__text.u-green.u-bold
       %span.label.label--green Good
       Your DKIM record looks good!
@@ -56,10 +60,34 @@
       %span.label.label--orange Warning
       = @domain.dkim_error
 
-  %p.pageContent__text
-    You need to add a new TXT record with the name <b>#{@domain.dkim_record_name}</b>
-    with the following content.
-  %pre.codeBlock.u-margin= @domain.dkim_record
+  - if @domain.pending_dkim_key?
+    %p.pageContent__text.u-bold
+      %span.label.label--green Current
+      Keep this record exactly as it is
+    %p.pageContent__text
+      This is the record you already have published at <b>#{@domain.dkim_record_name}</b>.
+      It's signing your mail right now, so leave it in place.
+    %pre.codeBlock.u-margin= @domain.dkim_record
+
+    %p.pageContent__text.u-bold
+      %span.label.label--orange New
+      Add this record as well
+    %p.pageContent__text
+      Add a <b>second</b> TXT record with the name <b>#{@domain.pending_dkim_record_name}</b>
+      and the content below, alongside the record above rather than replacing it. Once we detect
+      it (checked hourly, or when you use the button above) we'll switch to signing with the new
+      key automatically. After the switch, keep the old record published while messages signed
+      with it may still be queued, held, or available for redelivery.
+    %pre.codeBlock.u-margin= @domain.pending_dkim_record
+    %p.pageContent__text
+      = link_to "Cancel new DKIM key", [:cancel_dkim_regeneration, organization, @server, @domain], :remote => true, :method => :post, :data => { :confirm => "Are you sure? The new key will be discarded and your existing key will remain active." }, :class => 'button button--neutral'
+  - else
+    %p.pageContent__text
+      You need to add a new TXT record with the name <b>#{@domain.dkim_record_name}</b>
+      with the following content.
+    %pre.codeBlock.u-margin= @domain.dkim_record
+    %p.pageContent__text
+      = link_to "Regenerate DKIM key", [:regenerate_dkim, organization, @server, @domain], :remote => true, :method => :post, :data => { :confirm => "Are you sure you wish to generate a new DKIM key? You'll need to add a new DNS record before the new key can be used." }, :class => 'button button--neutral'
 
   %h3.pageContent__subTitle Return Path
   - if @domain.return_path_status == 'OK'
diff --git a/config/routes.rb b/config/routes.rb
index 11ef179..9e94812 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -12,12 +12,16 @@ Rails.application.routes.draw do
       match :verify, on: :member, via: [:get, :post]
       get :setup, on: :member
       post :check, on: :member
+      post :regenerate_dkim, on: :member
+      post :cancel_dkim_regeneration, on: :member
     end
     resources :servers, except: [:index] do
       resources :domains, only: [:index, :new, :create, :destroy] do
         match :verify, on: :member, via: [:get, :post]
         get :setup, on: :member
         post :check, on: :member
+        post :regenerate_dkim, on: :member
+        post :cancel_dkim_regeneration, on: :member
       end
       resources :track_domains do
         post :toggle_ssl, on: :member
diff --git a/db/migrate/20260903100000_add_pending_dkim_fields_to_domains.rb b/db/migrate/20260903100000_add_pending_dkim_fields_to_domains.rb
new file mode 100644
index 0000000..b46ad93
--- /dev/null
+++ b/db/migrate/20260903100000_add_pending_dkim_fields_to_domains.rb
@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class AddPendingDKIMFieldsToDomains < ActiveRecord::Migration[7.0]
+
+  def change
+    add_column :domains, :pending_dkim_private_key, :text
+    add_column :domains, :pending_dkim_identifier_string, :string
+  end
+
+end
diff --git a/db/schema.rb b/db/schema.rb
index 9ab08fd..acf8c4d 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2024_03_11_205229) do
+ActiveRecord::Schema[7.1].define(version: 2026_09_03_100000) do
   create_table "additional_route_endpoints", id: :integer, charset: "utf8mb4", collation: "utf8mb4_general_ci", force: :cascade do |t|
     t.integer "route_id"
     t.string "endpoint_type"
@@ -99,6 +99,8 @@ ActiveRecord::Schema[7.0].define(version: 2024_03_11_205229) do
     t.integer "owner_id"
     t.string "dkim_identifier_string"
     t.boolean "use_for_any"
+    t.text "pending_dkim_private_key"
+    t.string "pending_dkim_identifier_string"
     t.index ["server_id"], name: "index_domains_on_server_id"
     t.index ["uuid"], name: "index_domains_on_uuid", length: 8
   end
diff --git a/doc/config/environment-variables.md b/doc/config/environment-variables.md
index e287459..06c092a 100644
--- a/doc/config/environment-variables.md
+++ b/doc/config/environment-variables.md
@@ -68,6 +68,7 @@ This document contains all the environment variables which are available for thi
 | `DNS_TRACK_DOMAIN` | String | The CNAME which tracking domains should be pointed to | track.postal.example.com |
 | `DNS_HELO_HOSTNAME` | String | The hostname to use in HELO/EHLO when connecting to external SMTP servers |  |
 | `DNS_DKIM_IDENTIFIER` | String | The identifier to use for DKIM keys in DNS records | postal |
+| `DNS_DKIM_KEY_SIZE` | Integer | The size (in bits) of RSA key to generate for DKIM signing (one of 1024, 2048, 3072 or 4096). Note that records for 2048-bit and larger keys exceed 255 characters and must be published as a split (multi-string) TXT record. | 2048 |
 | `DNS_DOMAIN_VERIFY_PREFIX` | String | The prefix to add before TXT record verification string | postal-verification |
 | `DNS_CUSTOM_RETURN_PATH_PREFIX` | String | The domain to use on external domains which points to the Postal return path domain | psrp |
 | `DNS_TIMEOUT` | Integer | The timeout to wait for DNS resolution | 5 |
diff --git a/doc/config/yaml.yml b/doc/config/yaml.yml
index a91c303..ded6630 100644
--- a/doc/config/yaml.yml
+++ b/doc/config/yaml.yml
@@ -147,6 +147,8 @@ dns:
   helo_hostname: 
   # The identifier to use for DKIM keys in DNS records
   dkim_identifier: postal
+  # The size (in bits) of RSA key to generate for DKIM signing (one of 1024, 2048, 3072 or 4096). Note that records for 2048-bit and larger keys exceed 255 characters and must be published as a split (multi-string) TXT record.
+  dkim_key_size: 2048
   # The prefix to add before TXT record verification string
   domain_verify_prefix: postal-verification
   # The domain to use on external domains which points to the Postal return path domain
diff --git a/lib/postal/config_schema.rb b/lib/postal/config_schema.rb
index 241d627..ec08fbc 100644
--- a/lib/postal/config_schema.rb
+++ b/lib/postal/config_schema.rb
@@ -365,6 +365,20 @@ module Postal
         default "postal"
       end
 
+      integer :dkim_key_size do
+        description "The size (in bits) of RSA key to generate for DKIM signing (one of 1024, 2048, 3072 or 4096). " \
+                    "Note that records for 2048-bit and larger keys exceed 255 characters and must be " \
+                    "published as a split (multi-string) TXT record."
+        default 2048
+        transform do |value|
+          unless value.nil? || [1024, 2048, 3072, 4096].include?(value)
+            raise Konfig::Error, "dns.dkim_key_size must be one of 1024, 2048, 3072 or 4096 (got #{value})"
+          end
+
+          value
+        end
+      end
+
       string :domain_verify_prefix do
         description "The prefix to add before TXT record verification string"
         default "postal-verification"
diff --git a/spec/factories/domain_factory.rb b/spec/factories/domain_factory.rb
index 4aeebe8..480461f 100644
--- a/spec/factories/domain_factory.rb
+++ b/spec/factories/domain_factory.rb
@@ -4,31 +4,33 @@
 #
 # Table name: domains
 #
-#  id                     :integer          not null, primary key
-#  server_id              :integer
-#  uuid                   :string(255)
-#  name                   :string(255)
-#  verification_token     :string(255)
-#  verification_method    :string(255)
-#  verified_at            :datetime
-#  dkim_private_key       :text(65535)
-#  created_at             :datetime
-#  updated_at             :datetime
-#  dns_checked_at         :datetime
-#  spf_status             :string(255)
-#  spf_error              :string(255)
-#  dkim_status            :string(255)
-#  dkim_error             :string(255)
-#  mx_status              :string(255)
-#  mx_error               :string(255)
-#  return_path_status     :string(255)
-#  return_path_error      :string(255)
-#  outgoing               :boolean          default(TRUE)
-#  incoming               :boolean          default(TRUE)
-#  owner_type             :string(255)
-#  owner_id               :integer
-#  dkim_identifier_string :string(255)
-#  use_for_any            :boolean
+#  id                             :integer          not null, primary key
+#  dkim_error                     :string(255)
+#  dkim_identifier_string         :string(255)
+#  dkim_private_key               :text(65535)
+#  dkim_status                    :string(255)
+#  dns_checked_at                 :datetime
+#  incoming                       :boolean          default(TRUE)
+#  mx_error                       :string(255)
+#  mx_status                      :string(255)
+#  name                           :string(255)
+#  outgoing                       :boolean          default(TRUE)
+#  owner_type                     :string(255)
+#  pending_dkim_identifier_string :string(255)
+#  pending_dkim_private_key       :text(65535)
+#  return_path_error              :string(255)
+#  return_path_status             :string(255)
+#  spf_error                      :string(255)
+#  spf_status                     :string(255)
+#  use_for_any                    :boolean
+#  uuid                           :string(255)
+#  verification_method            :string(255)
+#  verification_token             :string(255)
+#  verified_at                    :datetime
+#  created_at                     :datetime
+#  updated_at                     :datetime
+#  owner_id                       :integer
+#  server_id                      :integer
 #
 # Indexes
 #
diff --git a/spec/models/domain_spec.rb b/spec/models/domain_spec.rb
index 70dae6f..597d5dd 100644
--- a/spec/models/domain_spec.rb
+++ b/spec/models/domain_spec.rb
@@ -4,31 +4,33 @@
 #
 # Table name: domains
 #
-#  id                     :integer          not null, primary key
-#  dkim_error             :string(255)
-#  dkim_identifier_string :string(255)
-#  dkim_private_key       :text(65535)
-#  dkim_status            :string(255)
-#  dns_checked_at         :datetime
-#  incoming               :boolean          default(TRUE)
-#  mx_error               :string(255)
-#  mx_status              :string(255)
-#  name                   :string(255)
-#  outgoing               :boolean          default(TRUE)
-#  owner_type             :string(255)
-#  return_path_error      :string(255)
-#  return_path_status     :string(255)
-#  spf_error              :string(255)
-#  spf_status             :string(255)
-#  use_for_any            :boolean
-#  uuid                   :string(255)
-#  verification_method    :string(255)
-#  verification_token     :string(255)
-#  verified_at            :datetime
-#  created_at             :datetime
-#  updated_at             :datetime
-#  owner_id               :integer
-#  server_id              :integer
+#  id                             :integer          not null, primary key
+#  dkim_error                     :string(255)
+#  dkim_identifier_string         :string(255)
+#  dkim_private_key               :text(65535)
+#  dkim_status                    :string(255)
+#  dns_checked_at                 :datetime
+#  incoming                       :boolean          default(TRUE)
+#  mx_error                       :string(255)
+#  mx_status                      :string(255)
+#  name                           :string(255)
+#  outgoing                       :boolean          default(TRUE)
+#  owner_type                     :string(255)
+#  pending_dkim_identifier_string :string(255)
+#  pending_dkim_private_key       :text(65535)
+#  return_path_error              :string(255)
+#  return_path_status             :string(255)
+#  spf_error                      :string(255)
+#  spf_status                     :string(255)
+#  use_for_any                    :boolean
+#  uuid                           :string(255)
+#  verification_method            :string(255)
+#  verification_token             :string(255)
+#  verified_at                    :datetime
+#  created_at                     :datetime
+#  updated_at                     :datetime
+#  owner_id                       :integer
+#  server_id                      :integer
 #
 # Indexes
 #
@@ -162,6 +164,22 @@ describe Domain do
     it "generates a new dkim key" do
       expect { domain.generate_dkim_key }.to change { domain.dkim_private_key }.from(nil).to(match(/\A-+BEGIN RSA PRIVATE KEY-+/))
     end
+
+    it "generates a key of the configured size" do
+      domain.generate_dkim_key
+      expect(OpenSSL::PKey::RSA.new(domain.dkim_private_key).n.num_bits).to eq Postal::Config.dns.dkim_key_size
+    end
+
+    context "when a different key size is configured" do
+      before do
+        allow(Postal::Config.dns).to receive(:dkim_key_size).and_return(1024)
+      end
+
+      it "generates a key of that size" do
+        domain.generate_dkim_key
+        expect(OpenSSL::PKey::RSA.new(domain.dkim_private_key).n.num_bits).to eq 1024
+      end
+    end
   end
 
   describe "#dkim_key" do
@@ -183,6 +201,138 @@ describe Domain do
     end
   end
 
+  describe "#pending_dkim_key" do
+    context "when the domain has a pending DKIM key" do
+      let(:domain) { create(:domain, dkim_status: "OK") }
+
+      before do
+        domain.regenerate_dkim_key!
+      end
+
+      it "returns the pending dkim key as a OpenSSL::PKey::RSA" do
+        expect(domain.pending_dkim_key).to be_a OpenSSL::PKey::RSA
+        expect(domain.pending_dkim_key.to_s).to eq domain.pending_dkim_private_key
+      end
+    end
+
+    context "when the domain has no pending DKIM key" do
+      let(:domain) { create(:domain) }
+
+      it "returns nil" do
+        expect(domain.pending_dkim_key).to be_nil
+      end
+    end
+  end
+
+  describe "#regenerate_dkim_key!" do
+    context "when the current DKIM record is verified" do
+      let(:domain) { create(:domain, dkim_status: "OK") }
+
+      it "stores the new key as a pending key with a new identifier" do
+        domain.regenerate_dkim_key!
+        expect(domain.pending_dkim_private_key).to match(/\A-+BEGIN RSA PRIVATE KEY-+/)
+        expect(domain.pending_dkim_identifier_string).to be_present
+        expect(domain.pending_dkim_identifier_string).to_not eq domain.dkim_identifier_string
+      end
+
+      it "does not change the active key or status" do
+        expect { domain.regenerate_dkim_key! }.to_not(change { domain.reload.dkim_private_key })
+        expect(domain.dkim_status).to eq "OK"
+      end
+    end
+
+    context "when the current DKIM record is not verified" do
+      let(:domain) { create(:domain, dkim_status: "Missing", dkim_error: "No TXT records") }
+
+      it "replaces the active key immediately without creating a pending key" do
+        original_key = domain.dkim_key.to_s
+        domain.regenerate_dkim_key!
+        expect(domain.reload.dkim_private_key).to_not eq original_key
+        expect(domain.pending_dkim_private_key).to be_nil
+        expect(domain.dkim_key.to_s).to eq domain.dkim_private_key
+      end
+
+      it "resets the DKIM status" do
+        domain.regenerate_dkim_key!
+        expect(domain.dkim_status).to be_nil
+        expect(domain.dkim_error).to be_nil
+      end
+
+      context "when a key change was already in progress" do
+        # This happens when the active record breaks (so the status is no longer
+        # "OK") while a pending key is waiting to be activated. The immediate
+        # replacement supersedes the in-flight change, so the stale pending key
+        # must not be left behind to be activated later.
+        let(:domain) { create(:domain, dkim_status: "OK") }
+
+        before do
+          domain.regenerate_dkim_key!
+          domain.update!(dkim_status: "Missing", dkim_error: "No TXT records")
+        end
+
+        it "abandons the pending key" do
+          expect(domain.pending_dkim_private_key).to_not be_nil
+          domain.regenerate_dkim_key!
+          expect(domain.reload.pending_dkim_private_key).to be_nil
+          expect(domain.pending_dkim_identifier_string).to be_nil
+          expect(domain.pending_dkim_key).to be_nil
+        end
+
+        it "replaces the active key rather than promoting the pending one" do
+          stale_pending_key = domain.pending_dkim_private_key
+          original_key = domain.dkim_private_key
+          domain.regenerate_dkim_key!
+          expect(domain.reload.dkim_private_key).to_not eq original_key
+          expect(domain.dkim_private_key).to_not eq stale_pending_key
+        end
+      end
+    end
+  end
+
+  describe "#activate_pending_dkim_key" do
+    context "when there is no pending key" do
+      let(:domain) { create(:domain) }
+
+      it "does nothing" do
+        expect { domain.activate_pending_dkim_key }.to_not(change { domain.dkim_private_key })
+      end
+    end
+
+    context "when there is a pending key" do
+      let(:domain) { create(:domain, dkim_status: "OK") }
+
+      before do
+        domain.regenerate_dkim_key!
+      end
+
+      it "promotes the pending key and identifier and clears the pending values" do
+        pending_key = domain.pending_dkim_private_key
+        pending_identifier = domain.pending_dkim_identifier_string
+        domain.activate_pending_dkim_key
+        expect(domain.dkim_private_key).to eq pending_key
+        expect(domain.dkim_identifier_string).to eq pending_identifier
+        expect(domain.pending_dkim_private_key).to be_nil
+        expect(domain.pending_dkim_identifier_string).to be_nil
+        expect(domain.dkim_key.to_s).to eq pending_key
+        expect(domain.pending_dkim_key).to be_nil
+      end
+    end
+  end
+
+  describe "#cancel_pending_dkim_key!" do
+    let(:domain) { create(:domain, dkim_status: "OK") }
+
+    before do
+      domain.regenerate_dkim_key!
+    end
+
+    it "discards the pending key without changing the active key" do
+      expect { domain.cancel_pending_dkim_key! }.to_not(change { domain.reload.dkim_private_key })
+      expect(domain.pending_dkim_private_key).to be_nil
+      expect(domain.pending_dkim_identifier_string).to be_nil
+    end
+  end
+
   describe "#to_param" do
     context "when the domain has not been saved" do
       it "returns nil" do
@@ -274,6 +424,131 @@ describe Domain do
     end
   end
 
+  describe "#pending_dkim_record" do
+    context "when the domain has no pending DKIM key" do
+      it "returns nil" do
+        expect(domain.pending_dkim_record).to be_nil
+      end
+    end
+
+    context "when the domain has a pending DKIM key" do
+      let(:domain) { create(:domain, dkim_status: "OK") }
+
+      before do
+        domain.regenerate_dkim_key!
+      end
+
+      it "returns the DKIM record for the pending key" do
+        expect(domain.pending_dkim_record).to match(/\Av=DKIM1; t=s; h=sha256; p=.*;\z/)
+        expect(domain.pending_dkim_record).to_not eq domain.dkim_record
+      end
+
+      it "returns a record name using the pending identifier" do
+        expect(domain.pending_dkim_record_name).to eq "#{Postal::Config.dns.dkim_identifier}-#{domain.pending_dkim_identifier_string}._domainkey"
+      end
+    end
+  end
+
+  describe "#check_dkim_record" do
+    let(:domain) { create(:domain, dkim_status: "OK") }
+    let(:resolver) { instance_double(DNSResolver) }
+
+    before do
+      allow(domain).to receive(:resolver).and_return(resolver)
+    end
+
+    context "when there is a pending DKIM key" do
+      before do
+        domain.regenerate_dkim_key!
+      end
+
+      context "when the pending record has been published" do
+        it "activates and verifies the pending key from a single DNS response" do
+          responses = [[domain.pending_dkim_record], []]
+          allow(resolver).to receive(:txt).with("#{domain.pending_dkim_record_name}.#{domain.name}") { responses.shift }
+
+          pending_key = domain.pending_dkim_private_key
+          domain.check_dkim_record
+          expect(domain.dkim_private_key).to eq pending_key
+          expect(domain.pending_dkim_private_key).to be_nil
+          expect(domain.dkim_status).to eq "OK"
+          expect(domain.dkim_error).to be_nil
+          expect(responses).to eq([[]])
+        end
+      end
+
+      context "when multiple pending records have been published" do
+        before do
+          allow(resolver).to receive(:txt).with("#{domain.pending_dkim_record_name}.#{domain.name}").and_return([domain.pending_dkim_record, domain.pending_dkim_record])
+          allow(resolver).to receive(:txt).with("#{domain.dkim_record_name}.#{domain.name}").and_return([domain.dkim_record])
+        end
+
+        it "keeps the current key active and retains the pending key" do
+          original_key = domain.dkim_private_key
+          domain.check_dkim_record
+          expect(domain.dkim_private_key).to eq original_key
+          expect(domain.pending_dkim_private_key).to_not be_nil
+          expect(domain.dkim_status).to eq "OK"
+        end
+      end
+
+      context "when the pending record has not been published" do
+        before do
+          allow(resolver).to receive(:txt).with("#{domain.pending_dkim_record_name}.#{domain.name}").and_return([])
+          allow(resolver).to receive(:txt).with("#{domain.dkim_record_name}.#{domain.name}").and_return([domain.dkim_record])
+        end
+
+        it "keeps the current key active and retains the pending key" do
+          original_key = domain.dkim_private_key
+          domain.check_dkim_record
+          expect(domain.dkim_private_key).to eq original_key
+          expect(domain.pending_dkim_private_key).to_not be_nil
+          expect(domain.dkim_status).to eq "OK"
+        end
+      end
+
+      context "when the pending record does not match" do
+        before do
+          allow(resolver).to receive(:txt).with("#{domain.pending_dkim_record_name}.#{domain.name}").and_return(["v=DKIM1; t=s; h=sha256; p=something;"])
+          allow(resolver).to receive(:txt).with("#{domain.dkim_record_name}.#{domain.name}").and_return([domain.dkim_record])
+        end
+
+        it "keeps the current key active and retains the pending key" do
+          original_key = domain.dkim_private_key
+          domain.check_dkim_record
+          expect(domain.dkim_private_key).to eq original_key
+          expect(domain.pending_dkim_private_key).to_not be_nil
+          expect(domain.dkim_status).to eq "OK"
+        end
+      end
+    end
+
+    context "when there is no pending DKIM key" do
+      context "when the record is published" do
+        before do
+          allow(resolver).to receive(:txt).with("#{domain.dkim_record_name}.#{domain.name}").and_return([domain.dkim_record])
+        end
+
+        it "marks the record as OK" do
+          domain.check_dkim_record
+          expect(domain.dkim_status).to eq "OK"
+          expect(domain.dkim_error).to be_nil
+        end
+      end
+
+      context "when the record is missing" do
+        before do
+          allow(resolver).to receive(:txt).with("#{domain.dkim_record_name}.#{domain.name}").and_return([])
+        end
+
+        it "marks the record as missing" do
+          domain.check_dkim_record
+          expect(domain.dkim_status).to eq "Missing"
+        end
+      end
+    end
+  end
+
   describe "#return_path_domain" do
     it "returns the return path domain" do
       expect(domain.return_path_domain).to eq "#{Postal::Config.dns.custom_return_path_prefix}.#{domain.name}"
diff --git a/spec/requests/domains_controller_spec.rb b/spec/requests/domains_controller_spec.rb
new file mode 100644
index 0000000..4024bd2
--- /dev/null
+++ b/spec/requests/domains_controller_spec.rb
@@ -0,0 +1,144 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "DomainsController", type: :request do
+  let(:user) { create(:user, admin: true) }
+  let(:organization) { create(:organization, owner: user) }
+  let(:server) { create(:server, organization: organization) }
+
+  before do
+    post "/login", params: { email_address: user.email_address, password: "passw0rd" }
+  end
+
+  describe "POST /org/:org/servers/:server/domains/:domain/regenerate_dkim" do
+    context "when the domain's DKIM record is verified" do
+      let(:domain) { create(:domain, owner: server, dkim_status: "OK") }
+
+      it "generates a pending DKIM key without changing the active key" do
+        original_key = domain.dkim_private_key
+        post "/org/#{organization.permalink}/servers/#{server.permalink}/domains/#{domain.uuid}/regenerate_dkim"
+        expect(response).to redirect_to(setup_organization_server_domain_path(organization, server, domain))
+        expect(flash[:notice]).to include("Keep the old DNS record published afterward")
+        expect(flash[:notice]).to include("queued, held, or available for redelivery")
+        expect(flash[:notice]).to_not include("you can remove the old record")
+        domain.reload
+        expect(domain.dkim_private_key).to eq original_key
+        expect(domain.pending_dkim_private_key).to_not be_nil
+      end
+    end
+
+    context "when the domain's DKIM record is not verified" do
+      let(:domain) { create(:domain, owner: server, dkim_status: "Missing") }
+
+      it "replaces the active key immediately" do
+        original_key = domain.dkim_private_key
+        post "/org/#{organization.permalink}/servers/#{server.permalink}/domains/#{domain.uuid}/regenerate_dkim"
+        expect(response).to redirect_to(setup_organization_server_domain_path(organization, server, domain))
+        domain.reload
+        expect(domain.dkim_private_key).to_not eq original_key
+        expect(domain.pending_dkim_private_key).to be_nil
+        expect(domain.dkim_status).to be_nil
+      end
+    end
+
+    context "when a key change is already in progress" do
+      let(:domain) { create(:domain, owner: server, dkim_status: "OK") }
+
+      before do
+        domain.regenerate_dkim_key!
+      end
+
+      it "refuses and leaves the pending key untouched" do
+        existing_pending_key = domain.pending_dkim_private_key
+        existing_pending_identifier = domain.pending_dkim_identifier_string
+        post "/org/#{organization.permalink}/servers/#{server.permalink}/domains/#{domain.uuid}/regenerate_dkim"
+        expect(response).to redirect_to(setup_organization_server_domain_path(organization, server, domain))
+        expect(flash[:alert]).to include("already in progress")
+        domain.reload
+        expect(domain.pending_dkim_private_key).to eq existing_pending_key
+        expect(domain.pending_dkim_identifier_string).to eq existing_pending_identifier
+      end
+
+      it "does not change the active key" do
+        expect do
+          post "/org/#{organization.permalink}/servers/#{server.permalink}/domains/#{domain.uuid}/regenerate_dkim"
+        end.to_not(change { domain.reload.dkim_private_key })
+      end
+    end
+  end
+
+  describe "POST /org/:org/domains/:domain/regenerate_dkim" do
+    let(:domain) { create(:domain, owner: organization, dkim_status: "OK") }
+
+    it "generates a pending DKIM key for an organization-owned domain" do
+      post "/org/#{organization.permalink}/domains/#{domain.uuid}/regenerate_dkim"
+      expect(response).to redirect_to(setup_organization_domain_path(organization, domain))
+      expect(domain.reload.pending_dkim_private_key).to_not be_nil
+    end
+  end
+
+  describe "POST /org/:org/servers/:server/domains/:domain/check with a pending DKIM key" do
+    let(:domain) { create(:domain, owner: server, dkim_status: "OK") }
+    let(:resolver) { instance_double(DNSResolver) }
+
+    before do
+      domain.regenerate_dkim_key!
+      allow(DNSResolver).to receive(:for_domain).and_return(resolver)
+      allow(resolver).to receive(:txt).with(domain.name).and_return([domain.spf_record])
+      allow(resolver).to receive(:mx).with(domain.name).and_return([])
+      allow(resolver).to receive(:cname).and_return([])
+    end
+
+    context "when the pending DKIM record has been published" do
+      before do
+        allow(resolver).to receive(:txt).with("#{domain.pending_dkim_record_name}.#{domain.name}").and_return([domain.pending_dkim_record])
+      end
+
+      it "activates the pending key and tells the user to retain the old record" do
+        old_record_name = domain.dkim_record_name
+        new_key = domain.pending_dkim_private_key
+        post "/org/#{organization.permalink}/servers/#{server.permalink}/domains/#{domain.uuid}/check"
+        expect(response).to redirect_to(setup_organization_server_domain_path(organization, server, domain))
+        expect(flash[:notice]).to include("new DKIM key is now active")
+        expect(flash[:notice]).to include(old_record_name)
+        expect(flash[:notice]).to include("queued, held, or available for redelivery")
+        expect(flash[:notice]).to_not include("can remove")
+        domain.reload
+        expect(domain.dkim_private_key).to eq new_key
+        expect(domain.pending_dkim_private_key).to be_nil
+        expect(domain.dkim_status).to eq "OK"
+      end
+    end
+
+    context "when the pending DKIM record has not been published" do
+      before do
+        allow(resolver).to receive(:txt).with("#{domain.pending_dkim_record_name}.#{domain.name}").and_return([])
+        allow(resolver).to receive(:txt).with("#{domain.dkim_record_name}.#{domain.name}").and_return([domain.dkim_record])
+      end
+
+      it "keeps the current key active and tells the user the new record wasn't verified" do
+        post "/org/#{organization.permalink}/servers/#{server.permalink}/domains/#{domain.uuid}/check"
+        expect(response).to redirect_to(setup_organization_server_domain_path(organization, server, domain))
+        expect(flash[:alert]).to include("couldn't verify the record for your new DKIM key")
+        domain.reload
+        expect(domain.pending_dkim_private_key).to_not be_nil
+        expect(domain.dkim_status).to eq "OK"
+      end
+    end
+  end
+
+  describe "POST /org/:org/servers/:server/domains/:domain/cancel_dkim_regeneration" do
+    let(:domain) { create(:domain, owner: server, dkim_status: "OK") }
+
+    before do
+      domain.regenerate_dkim_key!
+    end
+
+    it "discards the pending DKIM key" do
+      post "/org/#{organization.permalink}/servers/#{server.permalink}/domains/#{domain.uuid}/cancel_dkim_regeneration"
+      expect(response).to redirect_to(setup_organization_server_domain_path(organization, server, domain))
+      expect(domain.reload.pending_dkim_private_key).to be_nil
+    end
+  end
+end
diff --git a/spec/views/domains/index.html.haml_spec.rb b/spec/views/domains/index.html.haml_spec.rb
new file mode 100644
index 0000000..65b9eef
--- /dev/null
+++ b/spec/views/domains/index.html.haml_spec.rb
@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "domains/index", type: :view do
+  let(:user) { create(:user, admin: true) }
+  let(:organization) { create(:organization, owner: user) }
+  let(:domain) { create(:domain, owner: organization, dkim_status: "OK") }
+
+  before do
+    assign(:organization, organization)
+    assign(:domains, [domain])
+    assign(:server, nil)
+    without_partial_double_verification do
+      allow(view).to receive(:organization).and_return(organization)
+      allow(view).to receive(:current_user).and_return(user)
+      allow(view).to receive(:page_title).and_return(["Postal"])
+    end
+  end
+
+  context "when no key change is in progress" do
+    it "does not flag a pending DKIM key" do
+      render
+      expect(rendered).to_not include("New DKIM key pending")
+    end
+  end
+
+  context "when a key change is in progress" do
+    before do
+      domain.regenerate_dkim_key!
+    end
+
+    it "flags the domain as having a pending DKIM key" do
+      render
+      expect(rendered).to include("New DKIM key pending")
+    end
+
+    it "links the flag to the DNS setup page" do
+      render
+      expect(rendered).to include(setup_organization_domain_path(organization, domain))
+    end
+
+    it "still shows DKIM as OK because the active key is unaffected" do
+      render
+      expect(rendered).to include("domainList__check--ok")
+    end
+  end
+end
diff --git a/spec/views/domains/setup.html.haml_spec.rb b/spec/views/domains/setup.html.haml_spec.rb
new file mode 100644
index 0000000..6df9185
--- /dev/null
+++ b/spec/views/domains/setup.html.haml_spec.rb
@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "domains/setup", type: :view do
+  let(:user) { create(:user, admin: true) }
+  let(:organization) { create(:organization, owner: user) }
+  let(:domain) { create(:domain, owner: organization, dkim_status: "OK") }
+
+  before do
+    assign(:organization, organization)
+    assign(:domain, domain)
+    assign(:server, nil)
+    without_partial_double_verification do
+      allow(view).to receive(:organization).and_return(organization)
+      allow(view).to receive(:current_user).and_return(user)
+      allow(view).to receive(:page_title).and_return(["Postal"])
+    end
+  end
+
+  context "when no key change is in progress" do
+    it "shows the active record and offers regeneration" do
+      render
+      expect(rendered).to include(domain.dkim_record_name)
+      expect(rendered).to include(domain.dkim_record)
+      expect(rendered).to include("Regenerate DKIM key")
+      expect(rendered).to_not include("Key change in progress")
+      expect(rendered).to_not include("Cancel new DKIM key")
+    end
+  end
+
+  context "when a key change is in progress" do
+    before do
+      domain.regenerate_dkim_key!
+    end
+
+    it "shows both the current and the new record" do
+      render
+      expect(rendered).to include(domain.dkim_record_name)
+      expect(rendered).to include(domain.dkim_record)
+      expect(rendered).to include(domain.pending_dkim_record_name)
+      expect(rendered).to include(domain.pending_dkim_record)
+    end
+
+    it "labels which record is active and which must be added" do
+      render
+      expect(rendered).to include("Key change in progress")
+      expect(rendered).to include("Keep this record exactly as it is")
+      expect(rendered).to include("Add this record as well")
+    end
+
+    it "tells the user to add the new record rather than replace the old one" do
+      render
+      expect(rendered).to include("alongside the record above rather than replacing it")
+    end
+
+    it "does not describe the active record as one which needs adding" do
+      render
+      expect(rendered).to_not include("You need to add a new TXT record")
+    end
+
+    it "offers cancellation instead of another regeneration" do
+      render
+      expect(rendered).to include("Cancel new DKIM key")
+      expect(rendered).to_not include("Regenerate DKIM key")
+    end
+
+    it "does not claim the DKIM record is fully good" do
+      render
+      expect(rendered).to_not include("Your DKIM record looks good!")
+    end
+  end
+end