Although I know Postfix is available for mac68k NetBSD... I just don't know
about amavisd or spamassassin... but I would think as long as it works with
any version of NetBSD, and I can get the source, I should be able to compile
it for mac68k.
Postfix:
http://www.postfix.org
SpamAssassin:
http://www.spamassassin.org
Amavisd-new
http://www.ijs.si/software/amavisd/
Sources for all 3 are available. Amavisd and SpamAssassin are in Perl.