Unknown users are invalid or unrecognized email addresses in a sender's database. When you deploy an email to an email address that does not exist, a 5xx 5.x.x error message will appear in your Simple Mail Transfer Protocol (SMTP) bounce logs with a description such as:
- Unknown user
- No such user
- Invalid address
Unknown users might be email addresses that never existed, that were closed, or that were abandoned by the end user.
