I have on same server one virtual server (say domainA) that has external (MS Exchange) mail server and on this VS the feature “Mail for domain enabled” is unchecked. And I have another VS (domainB) that uses Postfix mail server and when mail@domainB tries to send email to mail@domainA it gets error
SMTP Error (550): Failed to add recipient “mail@dimainA” (5.1.1 mail@domainA: Recipient address rejected: User unknown in local recipient table).
This behavior is as if “Mail for domain enabled” is checked; assuming mail@domainA exists on the Exchange server.
Is it being delivered locally and therefore bouncing? Use the Virtualmin → Disable and Delete → Disable virtual server to disable the domainA virtual server for a minute and then try and send mail from mail@domainB to mail@domainA - does is get delivered successfully now that Virtualmin has nothing to do with domainA since you have just disabled it?
Else check the mail logs to see where the mail is attempted to be delivered.
it points correctly domaina-external.mail but for some reason the domainB on same vps is trying to send email to VPS mail server and not to external mail server. And it seams that somthing is on domainB virtual server as even I disable the domainA virtual server it still try to send it internally.
Any other email (outside the VPS) sent to domainA enters the mailbox (on external serever), and any other email sent from domainB leaves the server.