If you need a really useable web interface, Zimbra community edition might be an option. But it is quite ressource hungry. On the plus side it gives you a premi

If you need a really useable web interface, Zimbra community edition might be an option. But it is quite ressource hungry. On the plus side it gives you a premium CalDAV compatible shareable calendar server. Zafara might be a nice alternative to Zimbra. If you only need a mail server hot standby solution without web interface, then dovecot/postfix might be an option. Whichever solution you go for, feed it with the mails on the current Gmail server with imapsync or offlineimap. When you want to switch to your own server simply change the MX records of your domain.