Improve and finish the fallback to the In-Reply-To and References header for the reply-by-email feature

A few things to note:
- The IncomingEmail feature is now enabled even without a
  correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
  notifications to this thread are grouped in the thread by the email
  service that receives the notification
  (i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
  for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil

1 files changed, 5 insertions, 7 deletions

diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index e7331d88517..826e5f96fa1 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -110,6 +110,10 @@ class Notify < BaseMailer
 
       headers['Reply-To'] = address
 
+      fallback_reply_message_id = "<reply-#{reply_key}@#{Gitlab.config.gitlab.host}>".freeze
+      headers['References'] ||= ''
+      headers['References'] << ' ' << fallback_reply_message_id
+
       @reply_by_email = true
     end
 
@@ -121,17 +125,11 @@ class Notify < BaseMailer
   #
   # See: mail_answer_thread
   def mail_new_thread(model, headers = {})
-    headers['Message-ID'] = message_reply_id
-    headers['In-Reply-To'] = message_id(model)
-    headers['References'] = message_id(model)
+    headers['Message-ID'] = message_id(model)
 
     mail_thread(model, headers)
   end
 
-  def message_reply_id
-    Gitlab.config.incoming_email["address"].gsub("%{key}", reply_key)
-  end
-
   # Send an email that responds to an existing conversation thread,
   # with headers suitable for grouping by thread in email clients.
   #