diff --git a/.rubocop.yml b/.rubocop.yml
index 3c7e58f..25bb950 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -10,6 +10,7 @@ AllCops:
 # Always use double quotes
 Style/StringLiterals:
   EnforcedStyle: double_quotes
+  AutoCorrect: true
 
 # We prefer arrays of symbols to look like an array of symbols.
 # For example: [:one, :two, :three] as opposed to %i[one two three]
diff --git a/Gemfile b/Gemfile
index f5e76cf..31b5bef 100644
--- a/Gemfile
+++ b/Gemfile
@@ -48,9 +48,10 @@ end
 group :development do
   gem "annotate"
   gem "database_cleaner", require: false
-  gem "factory_bot_rails", "~> 4.0", require: false
+  gem "factory_bot_rails", require: false
   gem "rspec", require: false
   gem "rspec-rails", require: false
   gem "rubocop"
   gem "rubocop-rails"
+  gem "timecop"
 end
diff --git a/Gemfile.lock b/Gemfile.lock
index 56d7833..ada7fd9 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -89,7 +89,7 @@ GEM
       coffee-script-source
       execjs
     coffee-script-source (1.12.2)
-    concurrent-ruby (1.2.2)
+    concurrent-ruby (1.2.3)
     crass (1.0.6)
     database_cleaner (2.0.2)
       database_cleaner-active_record (>= 2, < 3)
@@ -104,15 +104,17 @@ GEM
     dotenv-rails (2.8.1)
       dotenv (= 2.8.1)
       railties (>= 3.2)
-    dynamic_form (1.1.4)
+    dynamic_form (1.3.1)
+      actionview (> 5.2.0)
+      activemodel (> 5.2.0)
     encrypto_signo (1.0.0)
     erubi (1.12.0)
     execjs (2.7.0)
-    factory_bot (4.11.1)
-      activesupport (>= 3.0.0)
-    factory_bot_rails (4.11.1)
-      factory_bot (~> 4.11.1)
-      railties (>= 3.0.0)
+    factory_bot (6.4.6)
+      activesupport (>= 5.0.0)
+    factory_bot_rails (6.4.3)
+      factory_bot (~> 6.4)
+      railties (>= 5.0.0)
     ffi (1.15.5)
     foreman (0.87.2)
     gelf (3.1.0)
@@ -125,7 +127,7 @@ GEM
       tilt
     hashie (5.0.0)
     highline (2.1.0)
-    i18n (1.12.0)
+    i18n (1.14.1)
       concurrent-ruby (~> 1.0)
     jquery-rails (4.5.1)
       rails-dom-testing (>= 1, < 3)
@@ -145,9 +147,9 @@ GEM
       activerecord
       kaminari-core (= 1.2.2)
     kaminari-core (1.2.2)
-    loofah (2.19.1)
+    loofah (2.22.0)
       crass (~> 1.0.2)
-      nokogiri (>= 1.5.9)
+      nokogiri (>= 1.12.0)
     mail (2.8.1)
       mini_mime (>= 0.1.1)
       net-imap
@@ -156,8 +158,7 @@ GEM
     marcel (1.0.2)
     method_source (1.0.0)
     mini_mime (1.1.2)
-    mini_portile2 (2.8.5)
-    minitest (5.18.0)
+    minitest (5.22.2)
     moonrope (2.0.2)
       deep_merge (~> 1.0)
       json
@@ -177,9 +178,6 @@ GEM
       activerecord (>= 4.0.0)
       activesupport (>= 4.0.0)
     nio4r (2.7.0)
-    nokogiri (1.16.2)
-      mini_portile2 (~> 2.8.2)
-      racc (~> 1.4)
     nokogiri (1.16.2-arm64-darwin)
       racc (~> 1.4)
     nokogiri (1.16.2-x86_64-linux)
@@ -190,7 +188,7 @@ GEM
     puma (6.4.2)
       nio4r (~> 2.0)
     racc (1.7.3)
-    rack (2.2.6.4)
+    rack (2.2.8)
     rack-test (2.1.0)
       rack (>= 1.3)
     rails (6.1.7.6)
@@ -208,11 +206,13 @@ GEM
       bundler (>= 1.15.0)
       railties (= 6.1.7.6)
       sprockets-rails (>= 2.0.0)
-    rails-dom-testing (2.0.3)
-      activesupport (>= 4.2.0)
+    rails-dom-testing (2.2.0)
+      activesupport (>= 5.0.0)
+      minitest
       nokogiri (>= 1.6)
-    rails-html-sanitizer (1.5.0)
-      loofah (~> 2.19, >= 2.19.1)
+    rails-html-sanitizer (1.6.0)
+      loofah (~> 2.21)
+      nokogiri (~> 1.14)
     railties (6.1.7.6)
       actionpack (= 6.1.7.6)
       activesupport (= 6.1.7.6)
@@ -220,7 +220,7 @@ GEM
       rake (>= 12.2)
       thor (~> 1.0)
     rainbow (3.1.1)
-    rake (13.0.6)
+    rake (13.1.0)
     rbtree (0.4.6)
     regexp_parser (2.7.0)
     resolv (0.2.2)
@@ -292,8 +292,9 @@ GEM
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
     temple (0.10.0)
-    thor (1.2.1)
+    thor (1.3.0)
     tilt (2.1.0)
+    timecop (0.9.8)
     timeout (0.3.2)
     turbolinks (5.2.1)
       turbolinks-source (~> 5.2)
@@ -306,11 +307,11 @@ GEM
     websocket-driver (0.7.6)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
-    zeitwerk (2.6.7)
+    zeitwerk (2.6.13)
 
 PLATFORMS
   arm64-darwin-22
-  ruby
+  arm64-darwin-23
   x86_64-linux
 
 DEPENDENCIES
@@ -330,7 +331,7 @@ DEPENDENCIES
   dynamic_form
   encrypto_signo
   execjs (~> 2.7, < 2.8)
-  factory_bot_rails (~> 4.0)
+  factory_bot_rails
   foreman
   gelf
   haml
@@ -356,6 +357,7 @@ DEPENDENCIES
   secure_headers
   sentry-rails
   sentry-ruby
+  timecop
   turbolinks (~> 5)
   uglifier (>= 1.3.0)
 
diff --git a/lib/postal/message_db/delivery.rb b/lib/postal/message_db/delivery.rb
index 177b7f9..dfe90cd 100644
--- a/lib/postal/message_db/delivery.rb
+++ b/lib/postal/message_db/delivery.rb
@@ -25,7 +25,7 @@ module Postal
         @attributes[name.to_s]
       end
 
-      def respond_to_missing?(name)
+      def respond_to_missing?(name, include_private = false)
         @attributes.key?(name.to_s)
       end
 
diff --git a/lib/postal/message_db/message.rb b/lib/postal/message_db/message.rb
index 7222640..c01ba3e 100644
--- a/lib/postal/message_db/message.rb
+++ b/lib/postal/message_db/message.rb
@@ -185,7 +185,7 @@ module Postal
         end
       end
 
-      def respond_to_missing?(name)
+      def respond_to_missing?(name, include_private = false)
         name = name.to_s.sub(/=\z/, "")
         @attributes.key?(name.to_s)
       end
diff --git a/lib/postal/smtp_server/client.rb b/lib/postal/smtp_server/client.rb
index 971995a..790e7dc 100644
--- a/lib/postal/smtp_server/client.rb
+++ b/lib/postal/smtp_server/client.rb
@@ -11,6 +11,12 @@ module Postal
       LOG_REDACTION_STRING = "[redacted]"
 
       attr_reader :logging_enabled
+      attr_reader :credential
+      attr_reader :ip_address
+      attr_reader :recipients
+      attr_reader :headers
+      attr_reader :state
+      attr_reader :helo_name
 
       def initialize(ip_address)
         @logging_enabled = true
@@ -55,19 +61,6 @@ module Postal
         end
       end
 
-      def sanitize_input_for_log(data)
-        if @password_expected_next
-          @password_expected_next = false
-          if data =~ /\A[a-z0-9]{3,}=*\z/i
-            return LOG_REDACTION_STRING
-          end
-        end
-
-        data = data.dup
-        data.gsub!(/(.*AUTH \w+) (.*)\z/i) { "#{::Regexp.last_match(1)} #{LOG_REDACTION_STRING}" }
-        data
-      end
-
       def finished?
         @finished || false
       end
@@ -137,7 +130,11 @@ module Postal
         @helo_name = data.strip.split(" ", 2)[1]
         transaction_reset
         @state = :welcomed
-        ["250-My capabilities are", Postal.config.smtp_server.tls_enabled? && !@tls ? "250-STARTTLS" : nil, "250 AUTH CRAM-MD5 PLAIN LOGIN"]
+        [
+          "250-My capabilities are",
+          Postal.config.smtp_server.tls_enabled? && !@tls ? "250-STARTTLS" : nil,
+          "250 AUTH CRAM-MD5 PLAIN LOGIN"
+        ].compact
       end
 
       def helo(data)
@@ -194,7 +191,7 @@ module Postal
           "334 UGFzc3dvcmQ6" # "Password:"
         end
 
-        data = data.gsub!(/AUTH LOGIN ?/i, "")
+        data = data.gsub(/AUTH LOGIN ?/i, "")
         if data.strip == ""
           @proc = username_handler
           "334 VXNlcm5hbWU6" # "Username:"
@@ -226,6 +223,7 @@ module Postal
             log "\e[33m WARN: AUTH failure for #{@ip_address}\e[0m"
             next "535 Denied"
           end
+
           grant = nil
           server.credentials.where(type: "SMTP").each do |credential|
             correct_response = OpenSSL::HMAC.hexdigest(CRAM_MD5_DIGEST, credential.key, challenge)
@@ -236,10 +234,12 @@ module Postal
             grant = "235 Granted for #{credential.server.organization.permalink}/#{credential.server.permalink}"
             break
           end
+
           if grant.nil?
             log "\e[33m WARN: AUTH failure for #{@ip_address}\e[0m"
             next "535 Denied"
           end
+
           grant
         end
 
@@ -458,6 +458,7 @@ module Postal
                 msg.mail_from = @mail_from
                 msg.raw_message = @data
                 msg.received_with_ssl = @tls
+                msg.bounce = 1
               end
             else
               # There's no return path route, we just need to insert the mesage
@@ -489,6 +490,19 @@ module Postal
         states.include?(@state)
       end
 
+      def sanitize_input_for_log(data)
+        if @password_expected_next
+          @password_expected_next = false
+          if data =~ /\A[a-z0-9]{3,}=*\z/i
+            return LOG_REDACTION_STRING
+          end
+        end
+
+        data = data.dup
+        data.gsub!(/(.*AUTH \w+) (.*)\z/i) { "#{::Regexp.last_match(1)} #{LOG_REDACTION_STRING}" }
+        data
+      end
+
     end
   end
 end
diff --git a/spec/factories/credential_factory.rb b/spec/factories/credential_factory.rb
new file mode 100644
index 0000000..5be9e3d
--- /dev/null
+++ b/spec/factories/credential_factory.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :credential do
+    server
+    name { "Example Credential" }
+    type { "API" }
+  end
+end
diff --git a/spec/factories/domain_factory.rb b/spec/factories/domain_factory.rb
index e128497..fe9ca66 100644
--- a/spec/factories/domain_factory.rb
+++ b/spec/factories/domain_factory.rb
@@ -38,7 +38,7 @@
 
 FactoryBot.define do
   factory :domain do
-    association :owner, factory: :user
+    association :owner, factory: :organization
     sequence(:name) { |n| "example#{n}.com" }
     verification_method { "DNS" }
     verified_at { Time.now }
diff --git a/spec/factories/http_endpoint_factory.rb b/spec/factories/http_endpoint_factory.rb
new file mode 100644
index 0000000..5fec9c8
--- /dev/null
+++ b/spec/factories/http_endpoint_factory.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :http_endpoint do
+    server
+    name { "HTTP endpoint" }
+    url { "https://example.com/endpoint" }
+    encoding { "BodyAsJSON" }
+    format { "Hash" }
+  end
+end
diff --git a/spec/factories/route_factory.rb b/spec/factories/route_factory.rb
new file mode 100644
index 0000000..045c487
--- /dev/null
+++ b/spec/factories/route_factory.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :route do
+    name { "test" }
+    mode { "Accept" }
+    spam_mode { "Mark" }
+
+    before(:create) do |route|
+      route.server ||= create(:server)
+      route.domain ||= create(:domain, owner: route.server)
+    end
+  end
+end
diff --git a/spec/factories/server_factory.rb b/spec/factories/server_factory.rb
index bd8c74e..3866001 100644
--- a/spec/factories/server_factory.rb
+++ b/spec/factories/server_factory.rb
@@ -48,5 +48,9 @@ FactoryBot.define do
     mode { "Live" }
     provision_database { false }
     sequence(:permalink) { |n| "server#{n}" }
+
+    trait :suspended do
+      suspended_at { Time.current }
+    end
   end
 end
diff --git a/spec/lib/postal/smtp_server/client/auth_spec.rb b/spec/lib/postal/smtp_server/client/auth_spec.rb
new file mode 100644
index 0000000..bdb271d
--- /dev/null
+++ b/spec/lib/postal/smtp_server/client/auth_spec.rb
@@ -0,0 +1,122 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+module Postal
+  module SMTPServer
+
+    describe Client do
+      let(:ip_address) { "1.2.3.4" }
+      subject(:client) { described_class.new(ip_address) }
+
+      before do
+        client.handle("HELO test.example.com")
+      end
+
+      describe "AUTH PLAIN" do
+        context "when no credentials are provided on the initial data" do
+          it "returns a 334" do
+            expect(client.handle("AUTH PLAIN")).to eq("334")
+          end
+
+          it "accepts the username and password from the next input" do
+            client.handle("AUTH PLAIN")
+            credential = create(:credential, type: "SMTP")
+            expect(client.handle(credential.to_smtp_plain)).to match(/235 Granted for/)
+          end
+        end
+
+        context "when valid credentials are provided on one line" do
+          it "authenticates and returns a response" do
+            credential = create(:credential, type: "SMTP")
+            expect(client.handle("AUTH PLAIN #{credential.to_smtp_plain}")).to match(/235 Granted for/)
+            expect(client.credential).to eq credential
+          end
+        end
+
+        context "when invalid credentials are provided" do
+          it "returns an error and resets the state" do
+            base64 = Base64.encode64("user\0pass")
+            expect(client.handle("AUTH PLAIN #{base64}")).to eq("535 Invalid credential")
+            expect(client.state).to eq :welcomed
+          end
+        end
+
+        context "when username or password is missing" do
+          it "returns an error and resets the state" do
+            base64 = Base64.encode64("pass")
+            expect(client.handle("AUTH PLAIN #{base64}")).to eq("535 Authenticated failed - protocol error")
+            expect(client.state).to eq :welcomed
+          end
+        end
+      end
+
+      describe "AUTH LOGIN" do
+        context "when no username is provided on the first line" do
+          it "requests the username" do
+            expect(client.handle("AUTH LOGIN")).to eq("334 VXNlcm5hbWU6")
+          end
+        end
+
+        context "when a username is provided on the first line" do
+          it "requests a password" do
+            username = Base64.encode64("xx")
+            expect(client.handle("AUTH LOGIN #{username}")).to eq("334 UGFzc3dvcmQ6")
+          end
+
+          it "authenticates and returns a response" do
+            credential = create(:credential, type: "SMTP")
+            username = Base64.encode64("xx")
+            password = Base64.encode64(credential.key)
+            expect(client.handle("AUTH LOGIN #{username}")).to eq("334 UGFzc3dvcmQ6")
+            expect(client.handle(password)).to match(/235 Granted for/)
+            expect(client.credential).to eq credential
+          end
+        end
+
+        context "when invalid credentials are provided" do
+          it "returns an error and resets the state" do
+            username = Base64.encode64("xx")
+            password = Base64.encode64("xx")
+            expect(client.handle("AUTH LOGIN #{username}")).to eq("334 UGFzc3dvcmQ6")
+            expect(client.handle(password)).to eq("535 Invalid credential")
+            expect(client.state).to eq :welcomed
+          end
+        end
+      end
+
+      describe "AUTH CRAM-MD5" do
+        context "when valid credentials are provided" do
+          it "authenticates and returns a response" do
+            credential = create(:credential, type: "SMTP")
+            result = client.handle("AUTH CRAM-MD5")
+            expect(result).to match(/\A334 [A-Za-z0-9=]+\z/)
+            challenge = Base64.decode64(result.split[1])
+            password = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new("md5"), credential.key, challenge)
+            base64 = Base64.encode64("#{credential.server.organization.permalink}/#{credential.server.permalink} #{password}")
+            expect(client.handle(base64)).to match(/235 Granted for/)
+            expect(client.credential).to eq credential
+          end
+        end
+
+        context "when no org/server matches the provided username" do
+          it "returns an error" do
+            client.handle("AUTH CRAM-MD5")
+            base64 = Base64.encode64("org/server password")
+            expect(client.handle(base64)).to eq "535 Denied"
+          end
+        end
+
+        context "when invalid credentials are provided" do
+          it "returns an error and resets the state" do
+            server = create(:server)
+            base64 = Base64.encode64("#{server.organization.permalink}/#{server.permalink} invalid-password")
+            client.handle("AUTH CRAM-MD5")
+            expect(client.handle(base64)).to eq("535 Denied")
+          end
+        end
+      end
+    end
+
+  end
+end
diff --git a/spec/lib/postal/smtp_server/client/data_spec.rb b/spec/lib/postal/smtp_server/client/data_spec.rb
new file mode 100644
index 0000000..d79fbb2
--- /dev/null
+++ b/spec/lib/postal/smtp_server/client/data_spec.rb
@@ -0,0 +1,86 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+module Postal
+  module SMTPServer
+
+    describe Client do
+      let(:ip_address) { "1.2.3.4" }
+      subject(:client) { described_class.new(ip_address) }
+
+      describe "DATA" do
+        it "returns an error if no helo" do
+          expect(client.handle("DATA")).to eq "503 HELO/EHLO, MAIL FROM and RCPT TO before sending data"
+        end
+
+        it "returns an error if no mail from" do
+          client.handle("HELO test.example.com")
+          expect(client.handle("DATA")).to eq "503 HELO/EHLO, MAIL FROM and RCPT TO before sending data"
+        end
+
+        it "returns an error if no rcpt to" do
+          client.handle("HELO test.example.com")
+          client.handle("MAIL FROM: test@example.com")
+          expect(client.handle("DATA")).to eq "503 HELO/EHLO, MAIL FROM and RCPT TO before sending data"
+        end
+
+        it "returns go ahead" do
+          route = create(:route)
+          client.handle("HELO test.example.com")
+          client.handle("MAIL FROM: test@test.com")
+          client.handle("RCPT TO: #{route.name}@#{route.domain.name}")
+          expect(client.handle("DATA")).to eq "354 Go ahead"
+        end
+
+        it "adds a received header for itself" do
+          route = create(:route)
+          client.handle("HELO test.example.com")
+          client.handle("MAIL FROM: test@test.com")
+          client.handle("RCPT TO: #{route.name}@#{route.domain.name}")
+          Timecop.freeze do
+            client.handle("DATA")
+            expect(client.headers["received"]).to include "from test.example.com (1.2.3.4 [1.2.3.4]) by postal.example.com with SMTP; #{Time.now.utc.rfc2822}"
+          end
+        end
+
+        describe "subsequent commands" do
+          let(:route) { create(:route) }
+          before do
+            client.handle("HELO test.example.com")
+            client.handle("MAIL FROM: test@test.com")
+            client.handle("RCPT TO: #{route.name}@#{route.domain.name}")
+            client.handle("DATA")
+          end
+
+          it "logs headers" do
+            client.handle("Subject: Test")
+            client.handle("From: test@test.com")
+            client.handle("To: test1@example.com")
+            client.handle("To: test2@example.com")
+            client.handle("X-Something: abcdef1234")
+            expect(client.headers["subject"]).to eq ["Test"]
+            expect(client.headers["from"]).to eq ["test@test.com"]
+            expect(client.headers["to"]).to eq ["test1@example.com", "test2@example.com"]
+            expect(client.headers["x-something"]).to eq ["abcdef1234"]
+          end
+
+          it "logs content" do
+            client.handle("Subject: Test")
+            client.handle("")
+            client.handle("This is some content for the message.")
+            client.handle("It will keep going.")
+            expect(client.instance_variable_get("@data")).to eq <<~DATA
+              Received: from test.example.com (1.2.3.4 [1.2.3.4]) by #{Postal.config.dns.smtp_server_hostname} with SMTP; #{Time.now.utc.rfc2822}\r
+              Subject: Test\r
+              \r
+              This is some content for the message.\r
+              It will keep going.\r
+            DATA
+          end
+        end
+      end
+    end
+
+  end
+end
diff --git a/spec/lib/postal/smtp_server/client/finished_spec.rb b/spec/lib/postal/smtp_server/client/finished_spec.rb
new file mode 100644
index 0000000..6438419
--- /dev/null
+++ b/spec/lib/postal/smtp_server/client/finished_spec.rb
@@ -0,0 +1,212 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+module Postal
+  module SMTPServer
+
+    describe Client do
+      let(:ip_address) { "1.2.3.4" }
+      let(:server) { GLOBAL_SERVER } # We'll use the global server instance for this
+      subject(:client) { described_class.new(ip_address) }
+
+      let(:credential) { create(:credential, server: server, type: "SMTP") }
+      let(:auth_plain) { credential&.to_smtp_plain }
+      let(:mail_from) { "test@example.com" }
+      let(:rcpt_to) { "test@example.com" }
+
+      before do
+        client.handle("HELO test.example.com")
+        client.handle("AUTH PLAIN #{auth_plain}") if auth_plain
+        client.handle("MAIL FROM: #{mail_from}")
+        client.handle("RCPT TO: #{rcpt_to}")
+      end
+
+      after do
+        server.message_db.provisioner.clean
+      end
+
+      describe "when finished sending data" do
+        context "when the data is larger than the maximum message size" do
+          it "returns an error and resets the state" do
+            allow(Postal.config.smtp_server).to receive(:max_message_size).and_return(1)
+            client.handle("DATA")
+            client.handle("a" * 1024 * 1024 * 10)
+            expect(client.handle(".")).to eq "552 Message too large (maximum size 1MB)"
+          end
+        end
+
+        context "when a loop is detected" do
+          it "returns an error and resets the state" do
+            client.handle("DATA")
+            client.handle("Received: from example1.com by #{Postal.config.dns.smtp_server_hostname}")
+            client.handle("Received: from example2.com by #{Postal.config.dns.smtp_server_hostname}")
+            client.handle("Received: from example1.com by #{Postal.config.dns.smtp_server_hostname}")
+            client.handle("Received: from example2.com by #{Postal.config.dns.smtp_server_hostname}")
+            client.handle("Subject: Test")
+            client.handle("From: #{mail_from}")
+            client.handle("To: #{rcpt_to}")
+            client.handle("")
+            client.handle("This is a test message")
+            expect(client.handle(".")).to eq "550 Loop detected"
+          end
+        end
+
+        context "when the email content is not suitable for the credential" do
+          it "returns an error and resets the state" do
+            client.handle("DATA")
+            client.handle("Subject: Test")
+            client.handle("From: invalid@krystal.uk")
+            client.handle("To: #{rcpt_to}")
+            client.handle("")
+            client.handle("This is a test message")
+            expect(client.handle(".")).to eq "530 From/Sender name is not valid"
+          end
+        end
+
+        context "when sending an outgoing email" do
+          let(:domain) { create(:domain, owner: server) }
+          let(:mail_from) { "test@#{domain.name}" }
+          let(:auth_plain) { credential.to_smtp_plain }
+
+          it "stores the message and resets the state" do
+            client.handle("DATA")
+            client.handle("Subject: Test")
+            client.handle("From: #{mail_from}")
+            client.handle("To: #{rcpt_to}")
+            client.handle("")
+            client.handle("This is a test message")
+            expect(client.handle(".")).to eq "250 OK"
+            queued_message = QueuedMessage.first
+            expect(queued_message).to have_attributes(
+              domain: "example.com",
+              server: server
+            )
+
+            expect(server.message(queued_message.message_id)).to have_attributes(
+              mail_from: mail_from,
+              rcpt_to: rcpt_to,
+              subject: "Test",
+              scope: "outgoing",
+              route_id: nil,
+              credential_id: credential.id,
+              raw_headers: kind_of(String),
+              raw_message: kind_of(String)
+            )
+          end
+        end
+
+        context "when sending a bounce message" do
+          let(:credential) { nil }
+          let(:rcpt_to) { "#{server.token}@#{Postal.config.dns.return_path}" }
+
+          context "when there is a return path route" do
+            let(:domain) { create(:domain, owner: server) }
+
+            before do
+              endpoint = create(:http_endpoint, server: server)
+              create(:route, domain: domain, server: server, name: "__returnpath__", mode: "Endpoint", endpoint: endpoint)
+            end
+
+            it "stores the message for the return path route and resets the state" do
+              client.handle("DATA")
+              client.handle("Subject: Bounce: Test")
+              client.handle("From: #{mail_from}")
+              client.handle("To: #{rcpt_to}")
+              client.handle("")
+              client.handle("This is a test message")
+              expect(client.handle(".")).to eq "250 OK"
+
+              queued_message = QueuedMessage.first
+              expect(queued_message).to have_attributes(
+                domain: Postal.config.dns.return_path,
+                server: server
+              )
+
+              expect(server.message(queued_message.message_id)).to have_attributes(
+                mail_from: mail_from,
+                rcpt_to: rcpt_to,
+                subject: "Bounce: Test",
+                scope: "incoming",
+                route_id: server.routes.first.id,
+                domain_id: domain.id,
+                credential_id: nil,
+                raw_headers: kind_of(String),
+                raw_message: kind_of(String),
+                bounce: true
+              )
+            end
+          end
+
+          context "when there is no return path route" do
+            it "stores the message normally and resets the state" do
+              client.handle("DATA")
+              client.handle("Subject: Bounce: Test")
+              client.handle("From: #{mail_from}")
+              client.handle("To: #{rcpt_to}")
+              client.handle("")
+              client.handle("This is a test message")
+              expect(client.handle(".")).to eq "250 OK"
+
+              queued_message = QueuedMessage.first
+              expect(queued_message).to have_attributes(
+                domain: Postal.config.dns.return_path,
+                server: server
+              )
+
+              expect(server.message(queued_message.message_id)).to have_attributes(
+                mail_from: mail_from,
+                rcpt_to: rcpt_to,
+                subject: "Bounce: Test",
+                scope: "incoming",
+                route_id: nil,
+                domain_id: nil,
+                credential_id: nil,
+                raw_headers: kind_of(String),
+                raw_message: kind_of(String),
+                bounce: true
+              )
+            end
+          end
+        end
+
+        context "when receiving an incoming email" do
+          let(:domain) { create(:domain, owner: server) }
+          let(:route) { create(:route, server: server, domain: domain) }
+
+          let(:credential) { nil }
+          let(:rcpt_to) { "#{route.name}@#{domain.name}" }
+
+          it "stores the message and resets the state" do
+            client.handle("DATA")
+            client.handle("Subject: Test")
+            client.handle("From: #{mail_from}")
+            client.handle("To: #{rcpt_to}")
+            client.handle("")
+            client.handle("This is a test message")
+            expect(client.handle(".")).to eq "250 OK"
+
+            queued_message = QueuedMessage.first
+            expect(queued_message).to have_attributes(
+              domain: domain.name,
+              server: server
+            )
+
+            expect(server.message(queued_message.message_id)).to have_attributes(
+              mail_from: mail_from,
+              rcpt_to: rcpt_to,
+              subject: "Test",
+              scope: "incoming",
+              route_id: route.id,
+              domain_id: domain.id,
+              credential_id: nil,
+              raw_headers: kind_of(String),
+              raw_message: kind_of(String)
+            )
+          end
+        end
+      end
+    end
+
+  end
+end
diff --git a/spec/lib/postal/smtp_server/client/helo_spec.rb b/spec/lib/postal/smtp_server/client/helo_spec.rb
new file mode 100644
index 0000000..85d513b
--- /dev/null
+++ b/spec/lib/postal/smtp_server/client/helo_spec.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+module Postal
+  module SMTPServer
+
+    describe Client do
+      let(:ip_address) { "1.2.3.4" }
+      subject(:client) { described_class.new(ip_address) }
+
+      describe "HELO" do
+        it "returns the hostname" do
+          expect(client.state).to eq :welcome
+          expect(client.handle("HELO: test.example.com")).to eq "250 #{Postal.config.dns.smtp_server_hostname}"
+          expect(client.state).to eq :welcomed
+        end
+      end
+
+      describe "EHLO" do
+        it "returns the capabilities" do
+          expect(client.handle("EHLO test.example.com")).to eq ["250-My capabilities are",
+                                                                "250 AUTH CRAM-MD5 PLAIN LOGIN"]
+        end
+
+        context "when TLS is enabled" do
+          it "returns capabilities include starttls" do
+            allow(Postal.config.smtp_server).to receive(:tls_enabled?).and_return(true)
+            expect(client.handle("EHLO test.example.com")).to eq ["250-My capabilities are",
+                                                                  "250-STARTTLS",
+                                                                  "250 AUTH CRAM-MD5 PLAIN LOGIN"]
+          end
+        end
+      end
+    end
+
+  end
+end
diff --git a/spec/lib/postal/smtp_server/client/mail_from_spec.rb b/spec/lib/postal/smtp_server/client/mail_from_spec.rb
new file mode 100644
index 0000000..b62de45
--- /dev/null
+++ b/spec/lib/postal/smtp_server/client/mail_from_spec.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+module Postal
+  module SMTPServer
+
+    describe Client do
+      let(:ip_address) { "1.2.3.4" }
+      subject(:client) { described_class.new(ip_address) }
+
+      describe "MAIL FROM" do
+        it "returns an error if no HELO is provided" do
+          expect(client.handle("MAIL FROM: test@example.com")).to eq "503 EHLO/HELO first please"
+          expect(client.state).to eq :welcome
+        end
+
+        it "resets the transaction when called" do
+          expect(client).to receive(:transaction_reset).and_call_original.at_least(3).times
+          client.handle("HELO test.example.com")
+          client.handle("MAIL FROM: test@example.com")
+          client.handle("MAIL FROM: test2@example.com")
+        end
+
+        it "sets the mail from address" do
+          client.handle("HELO test.example.com")
+          expect(client.handle("MAIL FROM: test@example.com")).to eq "250 OK"
+          expect(client.state).to eq :mail_from_received
+          expect(client.instance_variable_get("@mail_from")).to eq "test@example.com"
+        end
+      end
+    end
+
+  end
+end
diff --git a/spec/lib/postal/smtp_server/client/rcpt_to_spec.rb b/spec/lib/postal/smtp_server/client/rcpt_to_spec.rb
new file mode 100644
index 0000000..ee35abb
--- /dev/null
+++ b/spec/lib/postal/smtp_server/client/rcpt_to_spec.rb
@@ -0,0 +1,172 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+module Postal
+  module SMTPServer
+
+    describe Client do
+      let(:ip_address) { "1.2.3.4" }
+      subject(:client) { described_class.new(ip_address) }
+
+      describe "RCPT TO" do
+        let(:helo) { "test.example.com" }
+        let(:mail_from) { "test@example.com" }
+
+        before do
+          client.handle("HELO #{helo}")
+          client.handle("MAIL FROM: #{mail_from}") if mail_from
+        end
+
+        context "when MAIL FROM has not been sent" do
+          let(:mail_from) { nil }
+
+          it "returns an error if RCPT TO is sent before MAIL FROM" do
+            expect(client.handle("RCPT TO: no-route-here@internal.com")).to eq "503 EHLO/HELO and MAIL FROM first please"
+            expect(client.state).to eq :welcomed
+          end
+        end
+
+        it "returns an error if RCPT TO is not valid" do
+          expect(client.handle("RCPT TO: blah")).to eq "501 Invalid RCPT TO"
+        end
+
+        it "returns an error if RCPT TO is empty" do
+          expect(client.handle("RCPT TO: ")).to eq "501 RCPT TO should not be empty"
+        end
+
+        context "when the RCPT TO address is the system return path host" do
+          it "returns an error if the server does not exist" do
+            expect(client.handle("RCPT TO: nothing@#{Postal.config.dns.return_path}")).to eq "550 Invalid server token"
+          end
+
+          it "returns an error if the server is suspended" do
+            server = create(:server, :suspended)
+            expect(client.handle("RCPT TO: #{server.token}@#{Postal.config.dns.return_path}"))
+              .to eq "535 Mail server has been suspended"
+          end
+
+          it "adds a recipient if all OK" do
+            server = create(:server)
+            address = "#{server.token}@#{Postal.config.dns.return_path}"
+            expect(client.handle("RCPT TO: #{address}")).to eq "250 OK"
+            expect(client.recipients).to eq [[:bounce, address, server]]
+            expect(client.state).to eq :rcpt_to_received
+          end
+        end
+
+        context "when the RCPT TO address is on a host using the return path prefix" do
+          it "returns an error if the server does not exist" do
+            address = "nothing@#{Postal.config.dns.custom_return_path_prefix}.example.com"
+            expect(client.handle("RCPT TO: #{address}")).to eq "550 Invalid server token"
+          end
+
+          it "returns an error if the server is suspended" do
+            server = create(:server, :suspended)
+            address = "#{server.token}@#{Postal.config.dns.custom_return_path_prefix}.example.com"
+            expect(client.handle("RCPT TO: #{address}")).to eq "535 Mail server has been suspended"
+          end
+
+          it "adds a recipient if all OK" do
+            server = create(:server)
+            address = "#{server.token}@#{Postal.config.dns.custom_return_path_prefix}.example.com"
+            expect(client.handle("RCPT TO: #{address}")).to eq "250 OK"
+            expect(client.recipients).to eq [[:bounce, address, server]]
+            expect(client.state).to eq :rcpt_to_received
+          end
+        end
+
+        context "when the RCPT TO address is within the route domain" do
+          it "returns an error if the route token is invalid" do
+            address = "nothing@#{Postal.config.dns.route_domain}"
+            expect(client.handle("RCPT TO: #{address}")).to eq "550 Invalid route token"
+          end
+
+          it "returns an error if the server is suspended" do
+            server = create(:server, :suspended)
+            route = create(:route, server: server)
+            address = "#{route.token}@#{Postal.config.dns.route_domain}"
+            expect(client.handle("RCPT TO: #{address}")).to eq "535 Mail server has been suspended"
+          end
+
+          it "returns an error if the route is set to Reject mail" do
+            server = create(:server)
+            route = create(:route, server: server, mode: "Reject")
+            address = "#{route.token}@#{Postal.config.dns.route_domain}"
+            expect(client.handle("RCPT TO: #{address}")).to eq "550 Route does not accept incoming messages"
+          end
+
+          it "adds a recipient if all OK" do
+            server = create(:server)
+            route = create(:route, server: server)
+            address = "#{route.token}+tag1@#{Postal.config.dns.route_domain}"
+            expect(client.handle("RCPT TO: #{address}")).to eq "250 OK"
+            expect(client.recipients).to eq [[:route, "#{route.name}+tag1@#{route.domain.name}", server, { route: route }]]
+            expect(client.state).to eq :rcpt_to_received
+          end
+        end
+
+        context "when authenticated and the RCPT TO address is provided" do
+          it "returns an error if the server is suspended" do
+            server = create(:server, :suspended)
+            credential = create(:credential, server: server, type: "SMTP")
+            expect(client.handle("AUTH PLAIN #{credential.to_smtp_plain}")).to match(/235 Granted for /)
+            expect(client.handle("RCPT TO: outgoing@example.com")).to eq "535 Mail server has been suspended"
+          end
+
+          it "adds a recipient if all OK" do
+            server = create(:server)
+            credential = create(:credential, server: server, type: "SMTP")
+            expect(client.handle("AUTH PLAIN #{credential.to_smtp_plain}")).to match(/235 Granted for /)
+            expect(client.handle("RCPT TO: outgoing@example.com")).to eq "250 OK"
+            expect(client.recipients).to eq [[:credential, "outgoing@example.com", server]]
+            expect(client.state).to eq :rcpt_to_received
+          end
+        end
+
+        context "when not authenticated and the RCPT TO address is a route" do
+          it "returns an error if the server is suspended" do
+            server = create(:server, :suspended)
+            route = create(:route, server: server)
+            address = "#{route.name}@#{route.domain.name}"
+            expect(client.handle("RCPT TO: #{address}")).to eq "535 Mail server has been suspended"
+          end
+
+          it "returns an error if the route is set to Reject mail" do
+            server = create(:server)
+            route = create(:route, server: server, mode: "Reject")
+            address = "#{route.name}@#{route.domain.name}"
+            expect(client.handle("RCPT TO: #{address}")).to eq "550 Route does not accept incoming messages"
+          end
+
+          it "adds a recipient if all OK" do
+            server = create(:server)
+            route = create(:route, server: server)
+            address = "#{route.name}@#{route.domain.name}"
+            expect(client.handle("RCPT TO: #{address}")).to eq "250 OK"
+            expect(client.recipients).to eq [[:route, address, server, { route: route }]]
+            expect(client.state).to eq :rcpt_to_received
+          end
+        end
+
+        context "when not authenticated and RCPT TO does not match a route" do
+          it "returns an error" do
+            expect(client.handle("RCPT TO: nothing@nothing.com")).to eq "530 Authentication required"
+          end
+
+          context "when the connecting IP has an credential" do
+            it "adds a recipient" do
+              server = create(:server)
+              create(:credential, server: server, type: "SMTP-IP", key: "1.0.0.0/8")
+              address = "test@example.com"
+              expect(client.handle("RCPT TO: #{address}")).to eq "250 OK"
+              expect(client.recipients).to eq [[:credential, address, server]]
+              expect(client.state).to eq :rcpt_to_received
+            end
+          end
+        end
+      end
+    end
+
+  end
+end
diff --git a/spec/lib/postal/smtp_server/client_spec.rb b/spec/lib/postal/smtp_server/client_spec.rb
new file mode 100644
index 0000000..0c9261b
--- /dev/null
+++ b/spec/lib/postal/smtp_server/client_spec.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+module Postal
+  module SMTPServer
+
+    describe Client do
+      let(:ip_address) { "1.2.3.4" }
+      subject(:client) { described_class.new(ip_address) }
+    end
+
+  end
+end
diff --git a/spec/rails_helper.rb b/spec/rails_helper.rb
index 2e10886..ec9f85c 100644
--- a/spec/rails_helper.rb
+++ b/spec/rails_helper.rb
@@ -6,6 +6,7 @@ require File.expand_path("../config/environment", __dir__)
 require "rspec/rails"
 require "spec_helper"
 require "factory_bot"
+require "timecop"
 require "database_cleaner"
 
 DatabaseCleaner.allow_remote_database_url = true