I am in the process of migrating everything from server A to server B and that includes email services.
I have already set up server B to accept mail for our domain, user authentication is working and I also have a script which runs an rsync of Maildir data from server A to server B making the needed changes to accomodate for dovecot. Mail delivery for another test domain configured on server B works fine.
My problem now is how to sync & switch from server B to server A without (if possible) having mail delivered to both server while DNS caches expire.
Is there a way I can avoid the "inconsistency" period due to DNS TTL and have a seamless switchover to the new server?