Best failover strategy for e-mail servers on AWS to ensure high availability
You can clone the current mail server in another EC2 instance and run it as a backup MX server. Both servers database should be in sync with db level replication, and the disks should be in sync hrly with rsync/deltacopy. When the primary one goes down, sending email servers will automatically try to use secondary MX server, and users can still access old and new emails. When main server comes back, backup server will sync newest emails with main server again.