I dont think its a network problem. It is this .gitlab_incoming_email_secret file that gitlab expects. I don’t know what needs to be in there though. If you just touch /var/opt/gitlab/gitlab-rails/working/.gitlab_incoming_email_secret to create the file, then the exception does not happen. But the mails are just retrieved and deleted from the inbox.
I could help myself by configuring a forwarding for incoming mails. I wait until someone knows. what to do.
/var/opt/gitlab/gitlab-rails/working/ is the working dir in omnibus. At least on my instance. Check it on your side,