I needed to do this recently, when a broadband connection to one of my friends' networks went down. I configured my e-smith box to accept email for his domains in a store and forward fashion. As long as you have the ability to edit the DNS records for the domains, it should work fine.
All you need to do is make sure that the hostnames are in the /var/qmail/control/rcpthosts file.
I did this by adding a fragment in:
/etc/e-smith/templates-custom/var/qmail/control/20hostmxbackup
The content should be something like:
# - - - - - -
# 20hostmxbackup
#
# These are the hosts that I will accept mail for
#
foreigndomain1.com <-- substitute for your actual domain names.
foreigndomain2.com
#
EOF- - - - -
I expanded the template:
/sbin/e-smith/expand-template /var/qmail/control/rcpthosts
Then, you need to make sure the domains' DNS records (mine are at no-ip.com) indicate your backup server's address with a higher MX number than the main mail server for that domain.
The result is that mail received for any non-local domains will be forwarded by qmail at the first opportunity.
I was thinking that a panel snap-in would be good for this sort of thing, but then again, how often does anyone need this sort of thing?
regards...
Bob