[talison]

I used postfix-policyd for a while. I whitelisted the major email providers. You can also run the policy daemon in 'training mode' for a while.

http://www.policyd.org/tiki-index.php?page=Greylisting&s...

I still had the same problems others have mentioned though, where crappy registration systems were sending their confirmation mails directly from their PHP application, rather than relaying it through a smart MTA that actually understood the SMTP protocol and would do the retry.

Ultimately, too many users complained about mail delays and missing email. This is one of those scenarios where the cost of not delivering an important email may outweigh the benefits of blocking a few more invalid ones.