The most frequent problem permanent user faced is User Unknown. The account you are trying to mail no longer exists, probably because 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 below are all examples of User Unknown messages (Email errors):
host mail.domain.com[11.22.33.44] said: 550 No such user
<user@example.com>: host host.example.com said:
550 <user@example.com>… User unknown
<user@example.com>: host mail7.example.com said:
550 Requested action not taken: mailbox unavailable
<user@example.com>: host mail.example.com said:
550 5.1.1 <user@example.com> is not a valid mailbox
<user@example.com>: host example.com said:
550 Invalid recipient <user@example.com>
<user@example.com>: Sorry, no mailbox here by that name. (#5.1.1)
The message that you sent was undeliverable to the following:
user@example.com (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 try to contact the person you were trying to email by other means. Commonly, 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 any 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.
If you need assistance or have any further enquiries, feel free to contact our support team.
