Gmail is apparently experiencing technical issues. Several users have reported trouble sending mail to Gmail over the past hour. While we are unable to isolate it at this point we have contacted Gmail and are awaiting response.
The issue does not seem to be widespread, only about 10% of messages sent to Gmail terminate with the 5.x.x series error (permanent failure) and it is being issued by Gmail servers, not ExchangeDefender.
We have alerted Gmail and will update this blog post when they notify us of any problems being resolved on their end.