I noticed that our mattermost instance fails to send email notifications after an upgrade to 4.1.0, and the issue continues now with an upgrade to 4.2.0. In the mailer logs I see
==> /var/log/gitlab/mattermost/current <== 2017-10-19_17:45:55.89397 [2017/10/19 19:45:55 CEST] [EROR] api.post.send_notifications_and_forget.send.error%!(EXTRA email@example.com, *model.AppError=SendMail: utils.mail.send_mail.to_address.app_error, 550 5.7.1 Relaying denied)
The error also appears when trying to send a test email from the admin console.
While it could be that the upgrade coincided with the change of the way mailgun functions, I would find it surprising.
What are the possible origins of this error? Is there any way of getting more information for debugging?