Spammers vs Free Speech

A comment posted to my callbacks article suggests Google may have resorted to SMTP callbacks. AKA Sender Address Verification. Not only that, but their callback sender identifies itself as mx.google.com, and that name doesn't resolve in DNS. If it's true, everybody running an email server faces a choice.

1. We can figure out how to configure our servers to accept the bad HELO name from Google. But we can't just give mx.google.com a pass, because then the spammers would all use that name.
2. We can let Google's callbacks fail. Then Gmail users will refuse our email and they won't know why.

Maybe it's just a transient failure, and Google will publish an Address resource record in DNS for mx.google.com and fix it.