If you receive a user unknown bounce, and you have checked to make sure the address is correct, the next thing you should do is try to contact the person you were trying to email by other means. Frequently, people are not aware that they are bouncing email until someone else tells them. Also, do not delete the bounce message. It can help the user and their ISP resolve why they are bouncing mail more quickly.
If you do not have other means to contact the person who is bouncing mail, you probably have a dead email address. Try mailing them one more time (preferably at least a day later) in case it was a technical problem, but after that, stop using that email address. If you run a mailing list, and one of your subscribers starts bouncing mail with this error, remove them from the list.