Unable to send an email to a domain hosted in Plesk: mailbox unavailable or not local

Symptoms

• After sending an email from one domain to another (both hosted on the same Plesk server), the email is not delivered and a bounce message is received: 

  Delivery to the following recipient failed permanently:
  [email protected]
  Technical details of permanent failure:
  Google tried to deliver your message, but it was rejected by the server for the recipient domain example.com by mail.example.com [203.0.113.2].

  ---

  An error occurred while sending mail.
  550 Requested action not taken: mailbox unavailable or not local
  Please check the message recipient [email protected] and try again.

• Mail for the recipient domain is served externally (for example, at Gmail or Office 365).

Cause

Mail service of the recipient domain is enabled in Plesk/MailEnable.

Resolution

Disable mail service for the domain, mail of which is served externally, in Plesk at Domains > example.com > Mail tab > Mail Settings.