What does the message 'address rejected: user unknown in virtual alias table' mean?

[Rain Vaine]

Hello experts,
We encounter a scenario wherein the outbound email is not being sent to a particular user. When checking on the logs, it mentioned about 'Bouncing email address' and when I traced to Bouncing email address logs, I can see that it was undelivered. Upon further checking on the additional details column, the message 'address rejected: user unknown in virtual alias table' can be see:

Does anyone of you know of that meaning? Additionally the status in the bounce email address logs is 5.1.1. Do you have any idea on that status?
Is the problem still related to ServiceNow or on the mail server itself?
When checking on the email of the user, the user is existing in our instance, and the user were actually able to send inbound emails. The only problem is receiving the email.

Regards,
Vaine

[Mark Manders]

After the email leaves ServiceNow it's no longer a ServiceNow issue. Checking on the error message I see a lot about aliases and issues with how the domain is setup, but none point to an issue in ServiceNow.

[Rain Vaine]

Is it possible that the issue is with the SMTP server?
We just checked by trying to send on the email outside of ServiceNow, like Gmail and it was receiving mail as normal. 

[Mark Manders]

Does the email log say that the email has been sent?