My problem was that whenever I sent an email from my boogly.net email (hosted by hostgator) to my email at celiacclub.com (email managed with Google Apps for domains), the message bounced, replying with this error:
550-Verification failed for <email>
550-No Such User Here
550 Sender verify failed (state 14).
It only happened for the boogly.net-to/from-celiacclub.com connection–emails from hotmail got [...]