SymptomsPostfix MTA is used in Parallels Plesk Panel.
A message cannot be delivered to the mail account email@example.com from Parallels Plesk Panel. The incoming message is bounced with the following notification:
126.96.36.199 #<188.8.131.52 #5.0.0 smtp; 550 <firstname.lastname@example.org>: Recipient address rejected: User unknown in virtual mailbox table> #SMTP#
The following error is displayed in the maillog on the Parallels Plesk Panel server:
Jun 18 02:45:03 plesk.hostname postfix/pipe: 1A9DC8AEB2: to=<email@example.com>, relay=plesk_virtual, delay=19138, status=deferred (temporary failure. Command output: 4.3.5 Unable to select desired mailbox )
What does the error mean?
CauseThe message "Command output: 4.3.5 Unable to select desired mailbox" is a Postfix reject notification. It means that the incoming message was rejected because the mail account firstname.lastname@example.org does not exist in Parallels Plesk Panel and the policy "Mail to nonexistent user" is set to "reject."
You may check the policy in the Parallels Plesk Panel GUI at Domains -> domain.tld -> Mail Accounts -> Mail Settings: Mail to nonexistent user.