I'm guessing that people who only accept encrypted mail are such a small minority, you're probably safe from most spammers if you do this. People who target yo

I'm guessing that people who only accept encrypted mail are such a small minority, you're probably safe from most spammers if you do this. People who target your company and employees specifically, maybe not. But even this could be made more difficult for spammers by only allowing access to your public key directory from trusted IP ranges. It's also a great idea from the standpoint of giving an encryption policy teeth. I'm thinking of a company where the official line is "We encrypt all our email," but then some IT screwup results in having half the company sending each other cleartext email for several quarters before anybody notices. If your mailservers reject unencrypted mail, the above scenario can't happen, because presumably people will notice when everyone's mail starts getting discarded, and it'll be fixed very quickly.