User Unknown is the common permanent problem an email user encounters. This means that the account you are trying to send an email to no longer exists. It could have been removed by the owner, it may have shut down or misspelled the username or domain. Before sending the email, make sure to check email spelling.
Examples of User Unknown messages:
|host mail.domain.com[18.104.22.168] said: 550 No such user
<email@example.com>: host host.example.com said:
<firstname.lastname@example.org>: host mail7.example.com said:
<email@example.com>: host mail.example.com said:
<firstname.lastname@example.org>: Sorry, no mailbox here by that name. (#5.1.1)
<email@example.com>: host example.com said:
The message that you sent was undeliverable to the following:
These errors usually come as a bounce back message. When you do receive one, double check the address if it is correct. If you can contact the person you were sending an email by other means and let them know about the bounce back you’ve received. Frequently, owners of the email address are not aware that their emails are bouncing back until they are informed about it. Please don’t delete the bounce back email, this can help the user and their provider resolve the issue as to why their emails are bouncing back.
If you run a mailing list, and one of your subscribers’ email is bouncing back, remove them from the list.