Quota Exceeded

I am really stuck. I have a virtualmin-based host with several virtual hosts. Quotas are unlimited everywhere. But one specific virtual host claims “Quota exceeded” when there is an email to be sent anywhere.
Also: if there is an Email being sent by Roundcube, which is installed globally for all virtual hosts in a different virtual host on the same server, it says: Reverse DNS not found.

Interestingly enough, the server itself HAS a Reverse DNS entry, but i doublechecked and found out, that the IP adress in question is MY IP adress, my local computer uses, not the IP of my server. Do i need to set anywhere, that my local mailserver does not validate the ip adress for a reverse dns by sending mails?

Where to look into in order to fix this? Checked everything in this post:


but am not any wiser.

Thanks for help, because everything else works. Strange, huh?

that one of the servers still had the postgrey_milter send restrictions, so i removed this, and webmail is working now. Still no-go on the Outlook IP thing :frowning: