Stay with me now...
- You have lots of websites
- You have shared hosting set up
- Your main website is on the same shared hosting
- You're using cpanel
- Your mail is actually pointed to google apps
- You receive email from all the domains that are also on google apps (ie when a client emails you)
- You don't receive joomla generated emails to your domain.
Check your DNS records on your domain. It is likely that for some reason, your server's php sendmail internal mechanism thinks your domain's email is deliverable to the server, when in fact it should be delivered to the MX at Google Apps.
In Cpanel (and if you've locked the feature in Cpanel, you may need to do it in WHM directly) when you change the MX Records, it should say "Remote Mail Server" or similar when it correctly detects the remote MX records being used. If it has Automatic or Local Mail Server, then the server is likely to be trying to deliver the mail locally instead.
Check in your cpanel too - if there's a system catchall account, you might find that it has them all there. You could also test it with a forwarder in cpanel for the address you're using, then send that off to your gmail account instead.
(Yes, did have this happen first time I had a remote MX setup, and then wasn't getting emails)