Mailer issues in Gitlab CE

We are currently trying to setup the Gitlab CE instance on FreeBSD 12.3 in a dedicated jail. In general we have it running, but we cannot overcome the emails notification features. We have supplied an email account with correct password for SMTP server. The problem is that the emails always get lost in the queue, for instance here I try to provoke a notification for a used because of a new issue to which he is assigned to:

The Gitlab CE instance was installed on the host following this guide. We cannot figure out what would be the cause of the error “RecordNotFound”, as the Issie gets created correctly.

I would appreciate all help!