We're about to move a client from a "regular" Courier Mail Server at another provider's server to Google Apps with us. So at the same time, we're switching their DNS records to "point" to our web server and Google's MX records.
How can we avoid e-mail getting lost in the transition? Should we shut down Courier beforehand so that it doesn't receive any e-mail until the DNS changes have propagated? What will happen to e-mail that is sent to Courier but not received?
Also, what about users who don't check their e-mail very often, how do we move their e-mail from Courier to Google Apps when they're on the same domain? Should we temporarily move Courier to another domain?
Best Answer
It takes time for DNS records to propogate - so there will be a period for which some clients think Google is your MX while others may try to use the old MX. Unless you've got a very unusual configuration, your old MTA will continue to accept emails for your domain until you switch it off / reconfigure it.
While you could just turn off your server, it's not necessary. You should reduce the TTL on your DNS records in advance if they are greater than an hour. Despite what Lucas says, you shouldn't expect the DNS changes to propogate in less than around 9 hours regardless of how low you set the TTL.
Set up fetchmail to retrieve the mail from the old system and forward it to the new.
You shouldn't be retrieving email (pop/imap) using the mx record. You can continue to access the old server using it's IP address.