The problem is that I already see enough legit mail hit the quarantine due to being HTML/multipart, suspected of being sent "direct-to-MX" due to Exchange's bizarre habit of not providing an audit trail via Received headers, etc.
Of course by the time you can inspect the body of a message, it's already sucked down a large chunk of your resources. Host type is useful in pre-filtering even before you go so far as to send the banner -- to get rid of or at least slow down the crap that you almost certainly know is on its way.
The biggest problem with email isn't that it doesn't work; the biggest problem with email is that there are so many vendors who simply refuse to implement SMTP properly.
I heartily agree! We have seen some laughable renditions of SMTP over the years. Regards, Ken -- MailChannels: Reliable Email Delivery (TM) | http://mailchannels.com -- Suite 203, 910 Richards St. Vancouver, BC, V6B 3C1, Canada Direct: +1-604-729-1741