Milter-greylist

> Some spam or virus SMPT engines use empty envelope addresses, too.

Sure. But for viruses is exist another technology to detect & delete 
(remove executable attach, filter mail by helo = your domain, 
antivirus plugins...)
Many viruses come from post servers & greylisting don't fight with 
such mails :)
He succesful stop mail only from "zombi" computer, dialup & cable 
users, that spread spam&viruses.

We receive is about 500,000 spam letters in month (kill it) + 35,000 
viruses (kill it)
Mail with sender = <> ~ 1000-1500, they filter later, if that needed.

> Milter Greylist isn't blocking it, it's delaying it.

I see :) But i don't want to block my outbound mail to system that 
used callback sender verification with NULL sender envelope.

Thank you.