I set up my second domain mx record to the mailcow server domain like in attachment. Tested over Gmail, it receive and send emails. If I try to send a mail from a domainXY which has nothing to do with mailcow, server, ip etc. I get the reply in Thunderbird “Recipient addresse rejected: User unknown in virtual mailbox table”.
Can find any entries in the logs. How it comes that Gmail works but other not? Would be happy about any advice.