service/forum.md

@@ -198,6 +198,21 @@ able to see those failures in:
 
 <https://forum.torproject.org/sidekiq/retries>
 
+### Dashboard warns about failed email jobs
+
+From time to time the Discourse dashboard will show a message like this:
+
+> There are 859 email jobs that failed. Check your app.yml and ensure that the
+> mail server settings are correct. See the failed jobs in Sidekiq.
+
+In the Sidekiq logs, all the failed job error messages contain
+`Recipient address rejected: Domain not found `.
+
+This is caused by some user's email domain going dark, but Discourse keeps
+trying to send them the daily email digest. See the [Mass-disable email
+digests](#mass-disable-email-digests) section for instructions how to disable
+the automatic email digests for these users.
+
 ### Upgrade failure
 
 When upgrading using the web interface, it's possible for the process to fail