Add details of service desk custom mailbox on .com

doc/user/gitlab_com/index.md

@@ -39,6 +39,13 @@ and has its own dedicated IP address (`192.237.158.143`).
 
 The IP address for `mg.gitlab.com` is subject to change at any time.
 
+### Service Desk custom mailbox
+
+On GitLab.com there's a mailbox configured for Service Desk with the email adress:
+`contact-project+%{key}@incoming.gitlab.com`. To use this mailbox, configure the
+[custom suffix](../project/service_desk.md#configuring-a-custom-email-address-suffix) in project
+settings.
+
 ## Backups
 
 [See our backup strategy](https://about.gitlab.com/handbook/engineering/infrastructure/production/#backups).