In order to setup an email server you really should have at least 2 MX servers accepting mail in case one is down. It really really sucks when emails are delayed and the sender gets that delayed message.

The simplest self-hosting solution I've found is to setup 2 cheap VPS servers that accept mail via postfix (128mb, maybe 256mb if you want to virus scan at this point, a good idea), and they both can deliver email to an in-house server running roundcube or zarafa or whatever, when the internal server is up and running. As long as the VPSes are up, they hold email until the server inside your local network is up and running.

This means you bypass the very messy "secondary mx" issues, as you only have one internal server to worry about that accepts email arbitrarily from the 2 public server, and you can backup your internal server just like a normal PC.