When running spamass-milter with postfix (before queue) it creates fake received headers because at this point are no local received headers pressent. Release 0.3.1 has some typos in the header templates. The result are headers that are not parseable by spamassasing (UNPARSEABLE_RELAY) which leads to high scoring of correctly relayed mails and false positives.
A patch can be found there: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=510665
+*spamass-milter-0.3.1-r4 (08 Jun 2011)
+ 08 Jun 2011; Eray Aslan <firstname.lastname@example.org> +spamass-milter-0.3.1-r4.ebuild,
+ +files/spamass-milter-auth_users.patch, +files/spamass-milter-header.patch,
+ Security bump - bug #310049. Don't spam check authenticated users - bug
+ #265621. Fix received headers - bug #264304