If you have a large number of items in a single mailbox in Entourage, when you import into Apple Mail it will import no more than 6000 items from one mailbox.
So, if you have 17,000-odd items in your Sent Items (as a client of mine does) it takes three goes to import, delete, import, delete and import again.

It must be an arbitrary limit as is exactly 6000 items that it can import in one go…