Comment Re:RFC-Ignorant.org (Score 3, Informative) 195
There's even a solution for this kind of problem: It's called "recipient callout". The proxy SMTP Server will attempt a fake delivery attempt to the endpoint SMTP server before OKing a recipient. If it succeds, OK it, if it fails, deny it. Sure it costs some resources, but less than a bounce. And if you don't have enough resources to run an email server, don't run an email server.