Exchange rejects the message because it cannot find any valid recipient in its directory.
Exchange 2000 / 2003
- Verify your internet domain is added to the Default Policy under Recipients > Recipient Policies in Exchange System Manager.
- Verify the POPcon machine has relay rights in the virtual SMTP server connector properties.
Also check that user email addresses are properly assigned in Active Directory and that the domain in the recipient address matches the accepted domain in Exchange.