What is an unknown user?

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.


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