I want to setup a backup mail system on one of my spare RAQ4's.
If the primairy mailserver fails somehow, this RAQ4 collects the mail for my primairy server and resends it to the primairy machine when it comes back online.
The DNS part is clear to me, but I have found out that Qpopper does not support this 'backup' function. Qmail for instance has a file 'rcphosts' in which you can enter the domains for which Qmail will be the backup server.
Qpopper has nothing to do with collecting mail on your system, just sendmail.
All you need to do is set up a virtual site on the second RaQ, and tell it that it's allowed to accept mail for your other domain (add it to the mail server alias field).
Then make a secondary MX record for the virtual site (priority number greater than the priority on your primary MX record), pointing to the new virtual site.
If the primary goes down, all mail will be automatically routed to the secondary server. It will then turn into a "sending" server, attempting to deliver the mail to the original recipient. When your primary comes back up, the mail will be delivered.
You may want to edit the sendmail.cf file and increase the "permanent failure" time from the default of 7 days if you think your primary may ever be down for longer than that. Otherwise, if the secondary can't forward the mail on within 7 days, it will be permanently bounced to the sender.
I've seen some mention online that occasionally a manual 'ETRN' command is required to flush the queue for a domain, but as far as I know, it should all "just work."