Can we please create a solutions for the backup mail server to be a live system that syncs back and forth with the main server so if/when the main server goes down it doesn't only act as a repositry but as a full mail server that users can access to get and send e-mails? and when the main server comes back online they sync again and all mail that was received or sent during the downtime are replicated on both?

This is very important. I hope the community upvotes this.