the paid Google apps has an awesome feature that lets you configure it such that all outgoing mail is sent through a relay of your choice and that all incoming email should only be accepted by specific relays.

I'm using this to my advantage in that I'm keeping every mail ever sent to or from our Google apps accounts.

even better: the messages are kept in maildir format that's ready to be served over IMAP if Google really screws up.

Being able to do this configuration was the condition for me to even consider Google apps for serious purpose.