Add information on ignored headers for Service Desk

doc/administration/incoming_email.md

@@ -62,8 +62,8 @@ can reserve your catch-all mailbox for other purposes.
 
 ### Dedicated email address
 
-This solution is relatively simple to set up: you just need to create an email
-address dedicated to receive your users' replies to GitLab notifications. However,
+To set up this solution, you must create a dedicated email
+address to receive your users' replies to GitLab notifications. However,
 this method only supports replies, and not the other features of [incoming email](#incoming-email).
 
 ## Accepted headers
@@ -75,7 +75,7 @@ Email is processed correctly when a configured email address is present in one o
 - `Envelope-To` or `X-Envelope-To`
 
 In GitLab 14.6 and later, [Service Desk](../user/project/service_desk.md)
-also checks these additional headers.
+also checks accepted headers.
 
 Usually, the "To" field contains the email address of the primary receiver.
 However, it might not include the configured GitLab email address if:
@@ -84,6 +84,14 @@ However, it might not include the configured GitLab email address if:
 - The address was included when using "Reply all".
 - The email was forwarded.
 
+## Rejected headers
+
+To prevent unwanted issue creation from automatic email systems, GitLab ignores all incoming email
+containing the following headers:
+
+- `Auto-Submitted` with a value other than `no`
+- `X-Autoreply` with a value of `yes`
+
 ## Set it up
 
 If you want to use Gmail / Google Apps for incoming email, make sure you have

doc/user/project/service_desk.md

@@ -333,3 +333,10 @@ Note that:
 
 Behind the scenes, Service Desk works by the special Support Bot user creating issues. This user
 does not count toward the license limit count.
+
+## Troubleshooting Service Desk
+
+### Emails to Service Desk do not create issues
+
+Your emails might be ignored because they contain one of the
+[email headers that GitLab ignores](../../administration/incoming_email.md#rejected-headers).