This makes apps want to install ssmtp (which is blocked by postfix.)
Um, you might want to check out the mailer.eclass, which is inherited by this ebuild. The PROVIDE is right there.
Yes, this got fixed after I reported the bug: http://www.gentoo.org/cgi-bin/viewcvs.cgi/mail-mta/postfix/postfix-2.2.2-r2.ebuild?r1=1.1&r2=1.2