Mail to external mail service is delivered locally on Plesk server

Symptoms

• The domain example.com is hosted on Plesk server;

• Mail for example.com is hosted on an external mail server/service;

• Plesk emails notifications are not delivered;

• An attempt to send a message through Plesk server to a mailbox on example.com fails with

  # tail -f /usr/local/psa/var/log/maillog | grep ‘unknown in virtual mailbox’
  550 5.1.1 [email protected]: Recipient address rejected: User unknown in virtual mailbox table; [email protected] [email protected]

  And/or:

  example.com postfix-local[23040]: cannot chdir to mailname dir contact: No such file or directory
  example.com postfix-local[23040]: Unknown user: [email protected]

• The bounce message shows that there was a failed delivery attempt within the Plesk server, without an attempt to contact the external mail server that hosts the example.com mail service.

• In webmail, the following error can occur:

  Could not send message to SMTP server. Check you have access to send messages via the server and that all To/CC/BCC addresses are valid
  
  Error: SMTP Server rejected email. Returned:550 sorry, no mailbox here by that name. (#5.7.17)

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  Error SMTP (500): The following recipient could not be reached "[email protected]" (sorry, no mailbox here by that name. (#5.7.17)).

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  There was an error sending your message: Message could not be delivered - the address was not…