Exchange 2003 / SBS Issue - One user gets error '550 Requested action not taken: mailbox unavailable'

I have one specific user that cannot receive external email from anywhere. (Internal is fine)
External NDR is: 550 Requested action not taken: mailbox unavailable

I have checked the POP3 Connector - they are listed with the same setting that the rest of the staff are. I can't seem to figure out what other setting needs to be changed.

We are using the following:
Windows SBS 2003, Exchange 2003, POP3 Connector, SB SMTP Connector

I am a little unfamiliar with this type of setup, but it looks like they use both the POP3 connector and the SMTP connector to receive and send email via their web host.

Any ideas on where I could look?

Thanks,

E.D.
cuadminAsked:
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Alan HardistyCo-OwnerCommented:
If they are collecting their emails from a 3rd party, then it sounds like the email address hasn't been added to the 3rd party and that is why people get the unknown recipient error.

If mail went direct to the server there shouldn't be a problem.

Please check with the 3rd party and make sure the account / email address exists on their systems.

Alan
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