[ale] Backup MX basics

[Alex Carver]

I'm was pondering a backup MX for my server (I still haven't fixed its
clicky IDE drive) but it occured to me that I don't actually know how a
backup is configured to A) be the backup and B) make sure that it
delivers/synchronizes mail to the primary eventually.

Searching around online hasn't quite given me the basics, it just gives
me configuration parameters for different daemons without a full
explanation.  For example, I would assume that a backup has a more
liberal delivery timeout (e.g. ten days instead of four) but nothing
I've found makes any of that clear.

Suggestions for things to look at or just personal experiences?