Cheers.
When sending mail, how does it fail? Does everything look normal on the frontend (no error messages or so), but the mail do not arrive at their destination?
If so, maybe you need to set up SMTP. With Gmail and some other services you need to send with SMTP instead of PHP Mail (when sending, you got no visual feedback that the mail could not be delivered). I use http://wordpress.org/plugins/wp-smtp/ for this. Google Apps or Gmail-accounts are pretty common, so this is a easy way to fix the problem.
Else, see if you can come up with more details (if you got access to your logs, maybe you could find some clues).