Improve and finish the fallback to the In-Reply-To and References header for the reply-by-email feature2364-fallback-to-in-reply-to-header

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, 1 insertions, 1 deletions

diff --git a/config/gitlab.yml.example b/config/gitlab.yml.example
index 500b745f55e..fb1c3476f65 100644
--- a/config/gitlab.yml.example
+++ b/config/gitlab.yml.example
@@ -106,7 +106,7 @@ production: &base
     enabled: false
 
     # The email address including the `%{key}` placeholder that will be replaced to reference the item being replied to.
-    # The `%{key}` placeholder is added after the user part, after a `+` character, before the `@`.
+    # The placeholder can be omitted but if present, it must appear in the "user" part of the address (before the `@`).
     address: "gitlab-incoming+%{key}@gmail.com"
 
     # Email account username