The most common permanent user problem is, of course, User Unknown. The account you are trying to mail no longer exists - it may have been shut down, or you may have misspelled the username or domain (make sure you check email error messages carefully). The following are all examples of User Unknown messages:
<firstname.lastname@example.org>: host condor.prod.itd.earthlink.net said:
550 <email@example.com>... User unknown
<firstname.lastname@example.org>: host mail7.hotmail.com said:
550 Requested action not taken: mailbox unavailable
<email@example.com>: host mail.apc.net said:
550 5.1.1 <firstname.lastname@example.org> is not a valid mailbox
<email@example.com>: Sorry, no mailbox here by that name. (#5.1.1)
<firstname.lastname@example.org>: host hhh.net said:
550 Invalid recipient <email@example.com>
The message that you sent was undeliverable to the following:
firstname.lastname@example.org (user not found)
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.