I have an Exchange Server 2010 Enterprise. Two domains, ie: abc.com and abcfoundation.org. Both were pop3 and hosted offsite. The abcfoundation.org email addresses will be going away. The main email addresses will be abc.com. I have changed the mx record for the foundation email addresses to pint to the new server. They are receiving emails and can send emails to anyone outside the organization. When they send to abc.com, the email goes into the users exchange mailbox, instead of going out to the pop3 host mailboxes. I plan to change the other mx record later, but need the other group to receive their email in their pop3 accounts. Help is urgently needed.
The behaviour you are seeing is to be expected.
If you have accounts with the email addresses on them, then Exchange will deliver to those mailboxes. You cannot force Exchange to send the email out to an external account.
Therefore you have two options.
1. Remove the email addresses from the accounts and then configure Exchange to share the SMTP address space.
2. Stop using the POP3 mailboxes.
This isn't a problem unique to Exchange - all email servers will first attempt to match a mailbox with their known local accounts.
Simon Butler, Exchange MVP
Blog | Exchange Resources
Wade - Sembee is correct the SMTP Categorizer in Exchange first checks to see if you are the authoritative domain. If you are it will not route the mail off the exchange server by default. You need to change it from an Accepted Domain to a remote domain until you want the traffic to come back inside of your exchange organization.
Please understand these comments are made with limited information -- But this sounds like your issue.