Closes: gitlab#157

howto/gitlab.md

@@ -1771,6 +1771,12 @@ An alternative would be to encrypt outgoing mails with PGP/MIME. Some
 software that could do this was considered in the Schleuder
 retirement, see [TPA-RFC-41](policy/tpa-rfc-41-schleuder-retirement#role-key-and-home-made-schleuder).
 
+Note that this doesn't work with [external participants](https://docs.gitlab.com/ee/user/project/service_desk/external_participants.html), which can
+be used to CC arbitrary email addresses that do not have a GitLab
+account. If such an email gets added, confidential contents *will*
+leak through clear text email, see the discussion in
+[tpo/tpa/gitlab#157](https://gitlab.torproject.org/tpo/tpa/gitlab/-/issues/157).
+
 ## Issues
 
 [File][] or [search][] for issues in the [gitlab project][search].