[Dovecot] Date Received question

I migrated from Courier to Dovecot, and today I notice that all my
"date received" fields in my client (apple mail) show as the approx
date/time of the migration rather than when the emails were received
from the smtp server.

Is this expected behavior? I normally sort by received date rather
than sent -- since I don't care when someone sends something as much
as when i get it. But those fields are totally bogus now.

.tim