[ryeguy]

I'd imagine they do this to not tip off spammers when a message goes through. It's the same idea behind returning a 404 when trying to access a resource you don't have permission to, or not telling users if an account actually exists under an email when doing a password reset.

[b112]

It's simplistic for a spammer to tell if mail is getting delivered to end point. EG, just have a gmail account, and spam that account. Simple. Done.

Breaking reliable mail delivery for everyone, is inline with "there's no excuse, ever". It's inline with "making it worse", not better.

If you 250 accept, you deliver the email. Worst case, it ends up in a spam folder. You do not drop it on the floor. Ever. No excuse, no reason is valid here.

And I certainly won't accept "But it's so hard!", considering how easy it is to handle email, including SPAM, for everyone... until Google purposefully breaks it.