Add documentation on new email header

doc/administration/incoming_email.md

@@ -68,11 +68,16 @@ this method only supports replies, and not the other features of [incoming email
 
 ## Accepted headers
 
-Email is processed correctly when a configured email address is present in one of the following headers:
+> Accepting `Received` headers [introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81489) in GitLab 14.9 [with a flag](feature_flags.md) named `use_received_header_for_incoming_emails`. Enabled by default.
+
+Email is processed correctly when a configured email address is present in one of the following headers
+(sorted in the order they are checked):
 
 - `To`
+- `References`
 - `Delivered-To`
 - `Envelope-To` or `X-Envelope-To`
+- `Received`
 
 In GitLab 14.6 and later, [Service Desk](../user/project/service_desk.md)
 also checks accepted headers.
@@ -84,6 +89,9 @@ However, it might not include the configured GitLab email address if:
 - The address was included when using "Reply all".
 - The email was forwarded.
 
+The `Received` header can contain multiple email addresses. These are checked in the order that they appear.
+The first match is used.
+
 ## Rejected headers
 
 To prevent unwanted issue creation from automatic email systems, GitLab ignores all incoming email

doc/administration/reply_by_email.md

@@ -47,6 +47,8 @@ following headers, in this order:
 1. `References` header
 1. `Delivered-To` header
 1. `Envelope-To` header
+1. `X-Envelope-To` header
+1. `Received` header
 
 If it finds a reply key, it leaves your reply as a comment on
 the entity the notification was about (issue, merge request, commit...).