Incoming mail disappears with 2 domains

[Cooty]

Standalone SBS2000 server with exchange 2000 collecting email for two Internet domains, office-primary.com and office-secondary.com in a office shared by 2 companies. Mail is being collected by the pop3 connector via 2 ISP pop3 mailboxes.
Each domain works fine in isolation, but a single email sent or cc'd to users in both domains will not arrive at the office-secondary.com user. Notes:
1. The mail appears in both ISP pop3 mailboxes and is downloaded by the exchange server.
2. No ndr's are sent, locally or remotely.
3. No errors in event viewer
4. The second domain user's mail simply disappears.
Any ideas?

 

[Cooty]

Further diagnosis reveals that this appears to be down to the pop3 connector. It downloads the 2 mails ok, but doesn't pass the mail for the second domain to the smtp service, just ignores it. I have switched on maximum logging levels on all pop3 connector "services" but the mail is simply ignored.