Are you seeing these sent messages landing in the Junk Email/Spam folders in Gmail, or are they not getting there at all? If they're not getting there at all, do a search in your Delivery log for the gmail address you sent to and make sure to Display Related Traffic. That should return the SMTP session between your server and Google and you should then see why it failed. I hope that helps!
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com