I'm having an issue with Gravity Forms trying to send notifications to a Google Apps Account, not an @gmail.com account, but one that is using an @domainname.com account. I have checked the MX records with the domain host and they are correct.
I have even used a different email address, one that is a Google Apps Account and one that is a Gmail account and they both work. The Google Apps Account is also able to receive emails normaly.
This does seem like an issue where I have to contact my domain provider, but I'd thought I'd try here first to see what possible solution there is.