They also have a very clever greylisting approach -- it compares against senders in your address book as one point of data which I've never seen done before bec

They also have a very clever greylisting approach -- it compares against senders in your address book as one point of data which I've never seen done before because usually the spam filters I've run don't have access to that data and no open source greylist daemons do it to my knowledge