In response to my last entry about anti-spam measures and the difficulty of blocking SPAM at the SMTP protocol level I received a few responses. Brian May pointed out that the exiscan-acl feature of Exim allows such blocking, and Johannes Berg referred me to his web site http://johannes.sipsolutions.net/Projects for information on how he implemented Exim SPAM blocking at the SMTP level.
It seems that this is not possible in Postfix at this time. The only way I know of to do this in Postfix would be to have a SMTP proxy in front of the Postfix server that implements the anti-SPAM features. I have considered doing this in the past but not had enough time.
Also a comment on my blog criticises SORBS for blocking Tor (an anonymous Internet system). As I don’t want to receive anonymous email and none of the companies I work for want to receive it either this is something I consider a feature not a bug!