Exchange 2003 reporting Relaying Denied error for User Unknown

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Hello,

We have an Exchange 2003 server that forwards outbound e-mail to a sendmail
relay in our DMZ. An issue at a recipent's site caused a message to be
returned to sender with the "5.7.1 User Unknown" error. This error was logged
in the sendmail logs on the relay. The error was returned to the exchange
server and the sender was notified but it showed up in his mailbox as a
Relaying Denied error. Does anyone know why this might be happening?

Thanks,

Tom M
.

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