I was in the process of doing an emerge --world update when I noticed that ssmtp had been merged and clobbered my smtp daemon. I already had postfix installed, which provides virtual/mta, so ssmtp should not have been installed. I checked, and after deleting the mta ssmtp line from /etc/make.profiles/virtuals portage no longer tried to install ssmtp.
*** This bug has been marked as a duplicate of 393 ***