diff --git a/doc/source/gerrit.rst b/doc/source/gerrit.rst
index 8668c3f31f..a7d82af71b 100644
--- a/doc/source/gerrit.rst
+++ b/doc/source/gerrit.rst
@@ -500,6 +500,12 @@ delete the record with another call
 
    $ curl -XPOST  -u you.admin -i -H "Content-Type: application/json" -d '["https://login.ubuntu.com/+id/RaND0m"]' https://review.opendev.org/a/accounts/12345/external.ids:delete
 
+If the user has added email addresses, there may also be ``mailto:``
+identity entries for emails the user now wishes to use on their other
+account.  You should remove these with a ``:delete`` call as above.
+Note that ``username`` external-ids cannot be deleted (Gerrit will
+error), so new accounts can not reuse the username of old accounts.
+
 The user should now be able to add ``foo@new.com`` to their old
 account.