cc:Mail replies not being delivered to internet mailboxes

[Guest_imported]

Whenever a cc:Mail user tries to reply to an email from an internet email service, the message becomes non-deliverable at the cc:Mail MTA. The message is logged as (Unable to deliver message, All recipients are Unknown).

If a message is sent from a cc:Mail user to a user on the internet, it delivers just fine.

It only happens when trying to reply.

Any throughts or similar experiences would be very helpful.