Nothing, but it's also too slow, and pointless for them: It's easier for them to just send. Many of them clearly don't even care about checking response codes, or retry after greylisting, which is why these techniques works (greylisting cut our spam volume by about 90% off the bat).

But there's also no guarantee that the remote server will reject the address until after you've actually submitted a full e-mail, and send the final "." , exactly to make it even more expensive for spammers, in which case any attempt at checking short of actually sending an e-mail will fail.

Their suggestion may give you an indication, but if you try to rely on it you'll get a lot of pissed off users.