Having moved one of my domain's email services to Google Apps, I noticed that my contact form (formmail style script) no longer seemed to work. Emails coming from the form weren't getting spam tagged by Google... they simply wasn't getting there at all!
In the end I found a solution thanks to this thread, and for my own records, here it is:
REMOVE the domain from /etc/localdomains
ADD the domain to /etc/remotedomains (if it isn't already in there)
Then I restarted cPanel (service cpanel restart) although I'm not sure if that was necessary. Hope this helps anyone else that stumbles across this blog!
*Update*: Found another issue with Google Apps Mail and Formmail on another server, where the output of a formmail was going missing (due to being delivered locally rather than via Gmail). I blogged about it here.