if you just want a backup server to get mail, if your server goes down, all you need to do is set up 2 MX records for your domain.
ie - linuxquestions.org in MX 10 smtp.linuxquestions.org
linuxquestions.org in MX 50 smtp.yourisp.org
smtp.linuxquestions.org in A 192.xxx.xxx.xxx
this will sets priority on the Mail Exchanger, so that it will attempt the 10 first, then the 50 -- ie if 10 does work.
hope this answers your question.