Problem Description
After sending a message, you receive a bounce message that indicates "User unknown in virtual alias table". This means that the server you sent the message to does not have that email address configured. If you know the email address exists, then there's a good chance that the message was routing to the wrong server.
Problem Resolution
Most often this issue occurs when you've added the domain that you're sending to within your hosting account and forgot to disable mail for the domain even though mail is handled elsewhere.
For example, let's say you're sending to john@websavers.ca. You have websavers.ca added to your Plesk account, yet mail for websavers.ca is handled with an external provider like Google Apps rather than within your Websavers server.
To resolve this problem, disable mail for the domain on your local server.