"The e-mail address does not exist at the organization..." for email sent from outside.

Hey guys,

I have an interesting problem.  The organization I am currently working with has their exchange server using an POP3 connector to retreive email from an outside host, and connect it to their exchange mailbox store.  They want me to set it up to retrieve mail directly so that they can cancel their host.  The only problem is that their internal domain is completely different from their external domain.  Internally, it is abc.local and externally it is def.com.  Now, I have changed the MX records on their registrars DNS to be mail.def.com, and have a host record for mail.def.com pointing to their external IP.  I have also verified that the users have 2 SMTP addresses each (user@abc.local and user@def.com).  Port 110 and 25 are forwarded throught the firewall.  I am now stuck.  I know I am missing something, but for the life of me, cannot remember what it is.  When local users send to each other using either smtp address, it goes through fine, but when someone outside tries to send, they get a "The e-mail address does not exist at the organization..." error.  Any ideas?  Thanks in advance Experts.

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