I have had problems with mathematica on versions 7,8,9 using sendmail to a remote server or gmail. The best solution i have found so far is to configure postfix and send via localhost.
Clearing/deleting the postfix queue
sudo postsuper -d ALL
Sometimes starting postfix does not automatically start. Attached are the launch ctl commands to start postfix.
sudo launchctl unload /System/Library/LaunchDaemons/org.postfix.master.plist
sudo launchctl load /System/Library/LaunchDaemons/org.postfix.master.plist