Encode domain parts of all addresses in mailers as punycode

app/mailers/application_mailer.rb

@@ -1,10 +1,4 @@
 class ApplicationMailer < ActionMailer::Base
   append_view_path Rails.root.join('app', 'views', 'mailers')
   layout 'mailer'
-
-  def format(email)
-    local, host = email.split('@')
-    host = SimpleIDN.to_ascii(host)
-    "#{local}@#{host}"
-  end
 end

app/mailers/interceptors/punycode_interceptor.rb

@@ -0,0 +1,24 @@
+class PunycodeInterceptor
+  class << self
+    def delivering_email(message)
+      message.from = encode_addresses_as_punycode(message.from)
+      message.to = encode_addresses_as_punycode(message.to)
+      message.cc = encode_addresses_as_punycode(message.cc) if message.cc
+      message.bcc = encode_addresses_as_punycode(message.bcc) if message.bcc
+    end
+
+    private
+
+    def encode_addresses_as_punycode(addresses)
+      addresses.map do |address|
+        local_part, domain_part = address.split('@')
+        domain_part = encode_domain_part_as_punycode(domain_part)
+        [local_part, '@', domain_part].join
+      end
+    end
+
+    def encode_domain_part_as_punycode(domain_part)
+      SimpleIDN.to_ascii(domain_part)
+    end
+  end
+end

config/application.rb

@@ -92,6 +92,9 @@ module DomainNameRegistry
     }
     config.action_mailer.default_options = { from: ENV['action_mailer_default_from'] }
 
+    require "#{Rails.root}/app/mailers/interceptors/punycode_interceptor"
+    ActionMailer::Base.register_interceptor(PunycodeInterceptor)
+
     config.action_view.default_form_builder = 'DefaultFormBuilder'
     config.secret_key_base = Figaro.env.secret_key_base
   end

test/mailers/application_mailer_test.rb

@@ -14,4 +14,22 @@ class ApplicationMailerTest < ActiveSupport::TestCase
 
     assert_equal ['no-reply@registry.test'], email.from
   end
+
+  def test_encodes_address_fields_as_punycode
+    mailer = Class.new(ApplicationMailer) do
+      def test
+        # Empty block to avoid template rendering
+        mail(from: 'from@münchen.test', to: 'to@münchen.test', cc: 'cc@münchen.test',
+             bcc: 'bcc@münchen.test') {}
+      end
+    end
+
+    email = mailer.test
+    email.deliver_now
+
+    assert_equal ['from@xn--mnchen-3ya.test'], email.from
+    assert_equal ['to@xn--mnchen-3ya.test'], email.to
+    assert_equal ['cc@xn--mnchen-3ya.test'], email.cc
+    assert_equal ['bcc@xn--mnchen-3ya.test'], email.bcc
+  end
 end