We have users getting the following type of message when trying to send mail from their remote POP3 clients:

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'user@domain'. Subject 'hello', Account: 'mailserver.domain', Server: 'mailserver.domain', Protocol: SMTP, Server Response: '554 <user@domain>: Relay access denied', Port: 25, Secure(SSL): No, Server Error: 554, Error Number: 0x800CCC79