SpamAssassin can not drop a mail - it will always produce its output and not
producing it is treated as an error.
If you call SpamAssassin directly from within Postfix, you can use a
header check to discard the message on a high score.
Yeah, I'm surprised how many people answered "no" to this question. See
link:
http://wiki.apache.org/spamassassin/IntegratePostfixViaSpampd
That's not to say that it's wrong to use an additional milter e.g.
Amavis. You will likely get additional capabilities by doing so.
