I have a POP3 from ISP servicing about 1/2 of my organization.
I set up Exchange in our 30 person headquarters. I found a script that downloads from the POP3 on ISP to Exchange server. Everyone here is set up and working.
However, when I mail to our domain, the mail needs to goto the ISP's POP3 server, but instead looks on the Exchange address book and finds nothing.
Our internal domain is domainname.local. The company is domainname.com. I set up on reciepient policies, that our primary is domain.com. Domain.local is also on there.
Mail to other domains works fine. I need to find a way in exchange to force email for us to look up the MX record and goto the POP3 server.
If this is Exchange 2003 you could use the functionality on the SMTP Virtual Server to forward messages with unknown recipients to the ISP's mail server hosting the POP3. The setting is accessible in the properties for the SMTP Virtual Server.
This is sort of an inelegant solution, but it should do what you want.
(You could even make Contact objects in your AD for users who have accounts at the ISP w/ the POP3 such that they appear in the GAL for Outlook users. I shouldn't be encouraging this... >smile<)