See URL for info about bugs in old versions of clamassassin which are marked stable in portage. The most obvious problem is that clamassassin-1.2.3 doesn't work with clamscan from >=clamav-0.90. >=clamav-0.90 isn't in portage anymore.
+ 10 Jun 2011; Eray Aslan <eras@gentoo.org> -clamassassin-1.2.1.ebuild, + -clamassassin-1.2.2.ebuild, -clamassassin-1.2.3.ebuild: + Remove old - bug #300298 +