service/email.md

@@ -166,6 +166,21 @@ to Puppet in [tpo/tpa/team#40989][].
 [tpo/tpa/team#40988]: https://gitlab.torproject.org/tpo/tpa/team/-/issues/40988
 [tpo/tpa/team#40989]: https://gitlab.torproject.org/tpo/tpa/team/-/issues/40989
 
+If you get double OpenDKIM signatures, consider adding
+`receive_override_options=no_milters` to the server in `master.cf`, if
+already does some content filtering. 
+
+It's possible that some mail doesn't get signed when injected from
+Mailman, consider changing the `InternalHosts` to:
+
+    InternalHosts   eugeni.torproject.org,lists.torproject.org,127.0.0.1,::1,localhost # cargo-culted, to remove?
+
+If the signatures come from another server and cause problems (for
+example with Mailman), you can strip those with:
+
+    # strip incoming sigs, typically from submit-01 or other servers on mailman
+    RemoveOldSignatures yes
+
 ### SPF records
 
 TODO: summarize https://gitlab.torproject.org/tpo/tpa/team/-/issues/40981