Gitlab sends mails without a SMTP configuration

a strange thing was happend in our self hosted gitlab.
We adding an external user with an email address, the smtp server (hosting this email address) runs without authentication. After adding the user und adding the user to a repository, the user receives mails from gitlab. Smtp isn’t configured in our gitlab!

In our opinion this is a bug, not a feature!

GitLab information
Version: 15.6.2-ee
Revision: 08b668e8740

Thanks for any advice to solve this issue or to move it to the right place!

First of all, this it not possible, and Gitlab isn’t the culprit here. Most likely you have exim, postfix or sendmail daemons running on your Gitlab server and so this was used to send the email. I don’t configure email in gitlab.rb, but rather run postfix on my server and allow it to send emails for my domain with appropriate SPF records etc.

Okay, if this is the solution (I have to check).
Why is no other user (internal, domain, external) receiving mails from Gitlab?

Your SMTP server may be rejecting it, or your network may not have firewall rules to allow the traffic between that server and your mail server. Without seeing log files from either the Gitlab server or the mail server itself it’s hard to say why.

We got the solution.
GitLab will fallback on the Rails Framework default of using sendmail (configuring SMTP overrides the sendmail default).

To disable all mails: SMTP settings | GitLab

Thanks to all. We can close this topic.