But far more consistently. Most users have no idea how the Internet works, let alone hosting something as complex as email. They'd just stop using email and switch entirely to Facebook.
Also, never ever a 550 5.1.1 error.
On the same server... Was missing an email, went to look into other box that should have forwarded that. Got very suspicious looking email...
Good thing gmail wasn't completely stupid and didn't try to forward that one, getting in infinite loop...