Exchange 2000 losing emails?

We have a client running a Windows 2000 SBS Server & have encountered a weird problem. They have 2 domains registered for email, the same name – say domain1 – with a .com extension and a .co.uk extension.

The server collects email using the POP3 Connector. The issue appears when an email is sent to a@domain1.com and another person b@domain1.co.uk. For some reason the message will only reach b@domain1.co.uk.

We have redirected emails for the 2 domains to seperate mailboxes before being collected and confirmed that emails are reaching the ISP’s mailboxes before we collect them.

Any ideas?