I get a bounce back email whose contents are below and after researching, the solution is here.

Symptoms :

  • The domain example.com is hosted on Plesk server.
  • Mail for example.com is hosted on an external mail server/service.
  • An attempt to send a message through Plesk server to a mailbox on example.com fails with : Server error: ‘550 5.1.1 <email@example.com>: Recipient address rejected: User unknown in virtual mailbox table’

Cause :

  • Mail server treats example.com as a local domain and sends the mail locally.

Resolution :

  • Log in to Plesk.
  • Go to Domains > example.com > Mail Settings > Uncheck Activate mail service on domain and click OK.

Note : Plesk Obsidian Web Host Edition Version 18.0.23 Update #2

Please try again and good luck.