feat: new configuration system (and schema) (#2819)

spec/lib/received_header_spec.rb

@@ -12,7 +12,7 @@ describe ReceivedHeader do
       it "returns the correct string" do
         result = described_class.generate(nil, "testhelo", "1.1.1.1", :smtp)
         expect(result).to eq "from testhelo (hostname.com [1.1.1.1]) " \
-                             "by #{Postal.config.dns.smtp_server_hostname} " \
+                             "by #{Postal::Config.postal.smtp_hostname} " \
                              "with SMTP; #{Time.now.utc.rfc2822}"
       end
     end
@@ -21,7 +21,7 @@ describe ReceivedHeader do
       it "returns the correct string" do
         server = Server.new(privacy_mode: true)
         result = described_class.generate(server, "testhelo", "1.1.1.1", :smtp)
-        expect(result).to eq "by #{Postal.config.dns.smtp_server_hostname} " \
+        expect(result).to eq "by #{Postal::Config.postal.smtp_hostname} " \
                              "with SMTP; #{Time.now.utc.rfc2822}"
       end
     end
@@ -31,7 +31,7 @@ describe ReceivedHeader do
         server = Server.new(privacy_mode: false)
         result = described_class.generate(server, "testhelo", "1.1.1.1", :smtp)
         expect(result).to eq "from testhelo (hostname.com [1.1.1.1]) " \
-                             "by #{Postal.config.dns.smtp_server_hostname} " \
+                             "by #{Postal::Config.postal.smtp_hostname} " \
                              "with SMTP; #{Time.now.utc.rfc2822}"
       end
     end
@@ -40,7 +40,7 @@ describe ReceivedHeader do
       it "returns the correct string" do
         result = described_class.generate(nil, "web-ui", "1.1.1.1", :http)
         expect(result).to eq "from web-ui (hostname.com [1.1.1.1]) " \
-                             "by #{Postal.config.web.host} " \
+                             "by #{Postal::Config.postal.web_hostname} " \
                              "with HTTP; #{Time.now.utc.rfc2822}"
       end
     end