And it was highly vulnerable to denial of service attacks. It didn't check if the mailbox was valid during the envelope phase, so it would queue basicaly everything, then check the mailbox and send a bounce if necessary. Sending thousands of messages to random boxes (dictionary spam attack) would queue thousands of bounce messages that would be rejected by the (faked) sender domain, bringing the Qmail server to it's knees. As me how I know this...

Thing is, in most companies, is cheaper and more efficient to deal with a sporadic vulnerability than having your e-mail system DOSed every other week.

This is the kind of compromises that normal people and companies have to do all the time, but radicals and cryptopunks like DJB can't seem to understand. Sure, he's a brilliant mathematician and cryptographer, but his grasp of reality outside academia seems very flimsy, IMO.