Emails to recipients that are unknown in my Exchange/ActiveDirectory organization are rejected by my Exchange Server and seem to remain in POPcon's queue.

You need to disable Recipient Filtering in the Exchange System Manager to allow Exchange to send a non-delivery report back to senders trying to reach a non-existing mailbox instead of Exchange dropping the connection in such situations.

Open the Exchage System Manager and go to "Global Settings", "Message Delivery", "Recipient Filtering" tab, uncheck Filter recipients who are not in the Directory. That will solve the problem.

Exchange 2003 Server Global Settings

Exchange 2003 Message Delivery Properties

As an alternative you can use a new feature available in POPcon version 3.00: In the POP3 properties choose "re-route mail to unknown recipients" and have email to unknown recipients routed to your postmaster or administrator account.