[Discuss] Backup E-mail Server?

[Kent Borg]

Two days ago my e-mail server died. A powered USB hub apparently sent 
12V where only 5V should go, frying my redundant disks. And the 
Raspberry Pi 4.

I have replaced everything and it seems to be running again. I need a 
better recovery plan.

I used to have a backup server running in a different location, I need 
to do that again, but I am wonder how?

The previous version was crude: A completely independent and separate 
server, each server with its own MX record, one with a higher priority. 
This was annoying as a user, but it had the advantage of not having many 
single-points-of-failure. (DNS, me…)

In a few days, if this server is still running nicely, I plan to order a 
completely new set of hardware and set configure it. The question is how?

I see three general options:

1. As I did before, two servers, my wife and I have to check both, 
though mostly everything happens on the high priority server. Advantage: 
simple design.

2. As before, but with the extra safety of each server having a 
secondary set of users, and have each server automatically rsync 
Maildirs to the other's secondary users. (Or actually mail to the other 
backup user.) Advantage: The backup, once it is working, isn't likely to 
have one server kill the other, still pretty simple design.

3. Something where one server forwards e-mail to the other (but not same 
e-mail back again) to keep the two kinda in sync or perfectly in sync. 
Disadvantage: Tightly coupled, looks hard to set up, and prone to single 
systemic failure.

Suggestions? War stories?


Thanks,

-kb