Moving to another machine. How do I migrate email?

[utdrmac]

Hi all.
So here's the scoop. We have old machine A running cPanel 11 and new machine B running postfix/courier. Right now the MX record for our domain is pointing to machine A and all our customers are checking/logging into machine A for POP/IMAP.

I know that I need to change the DNS record for mail.ourdomain.com to point to machine B, but there is always that "dns lag" for propagation so some people's ISP's will still send emails to machine A instead of B.

What are the proper steps for migrating email this way? I can move the existing emails using rsync but how do I deal with DNS?

Thanks,
Matthew

[spector]

Prior to changing mx lower soa ttl, expire, refresh.
After changing mx deactivate exim on server A so new mails will not be accepted, and mail servers will try again in some time.