I get a bounce back email whose contents are below and after researching, the solution is here.
- 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 <firstname.lastname@example.org>: Recipient address rejected: User unknown in virtual mailbox table’
- Mail server treats example.com as a local domain and sends the mail locally.
- 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.