This message is sent back to you, the sender, from the SMTP server. The server is telling you that the "user" you tried to send e-mail to does not exist. The user (email@example.com) may have existed at some point in the past, but is no longer available.
As people open and close e-mail addresses, this will happen. Once removed from the system on which they previously had an e-mail account, all e-mail sent to them will bounce back.
A simple example would be sending e-mail to hih3720@maxterhost. Since we have no user setup under the login "hih3720", the e-mail will bounce back immediately. In order to correct this problem, you will need to contact the recipient and obtain their new e-mail address.
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