Adopting David's idea on beastie.frontfree.net
With more and more aaazzz’s received on beastie.frontfree.net, it makes me to seriously consider the possiblity to set up a set of early filter on postfix.
I finally added the following filter:
~^aaazzzaaazzzaaazzzaaazzzaaazzz(=0[Dd])?$~ DISCARD “mx2 aaazzz Junk”
This regular expression is derived from David’s (postmaster at FreeBSD.org) one, and it seems to work better than SpamAssassin because it does not bother to save copies of these mail.