Man, I wish I could program worth a crap... I'd sure like to get a plug-in that would do the same for ASSP...
assp doesn't seem to have (from a quick look at the docs) a command line program that takes a message... It seems to operate only from an smtp session to assp-spam or assp-notspam... Hence, you'd probably be best off by writing a small python (python because it has excellent mail handling modules) script that takes a mail on stdin, makes it into an attachment to assp-spam/assp-notspam and sends it off using /usr/sbin/sendmail.
It doesn't seem too hard. Maybe a half-day project for someone skilled enough. Maybe your best bet is to point the assp developers back at the dspam plugin and tell them that you've heard [what I said in above paragraph about not having a command line client] and that you'd like to see integration like the competition (dspam and now spamassassin) already has. Maybe they'll bite :P
Thanks for the input/evaluation - I'll give it a shot...
:)
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Best regards,
Charles