Complete Mail Server Backup / failsafe How to

[chrismfz]

Hello there.

Searching ways to make a failsafe/backup email server I found a few ways
like having 2,3,4,n mail servers as secondaries MX entries in a domain and
the domain in their allow list.

So if the main server eg mail.server.com fails, the email traffic will be redirected
to the next available in the MX entry list server. And when the 1st "main" server is online and available again, emails will be forwarded to the main server and a user/client
can retrieve them BUT with a delay (it depends when the 1st server will be back online).

I searched a lot but I couldnt find the solution I wanted.

Is there a way to have a second "main" server for the domains ?
For example, mail.server.com fails for good. There are the other email "slaves" from the MX list, but is there a way to retrieve those emails from a second mail2.server.com ?
So if there is a user chrismfz and cannot download his emails from mail.server.com for a x,y,z reason, he possibly could change the mail server to mail2.server.com and with the same user/pass to download his mails from there?