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
+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