For some reason today we're seeing hard email bounces from some gmail addresses. The hard bounces are triggering account invalidations. It would seem until we can either figure a work around or Google fix whatever is up the only option would be to use an alternate email address.
According to Google the issue is now resolved. https://www.google.com/appsstatus#hl=en&v=issue&sid=1&iid=a8b67908fadee664c68c240ff9f529ab Please note that because we were receiving a 500 class error message none of the failed messages will have another delivery attempt. For the most part they will have been thread update alerts and the like.