What I did emerge --unmerge mta # To get rid of ssmtp emerge sendmail # After this point ssmtp and procmail were installed (but not sendmail emerge --unmerge ssmtp # To get rid of ssmtp so I could install sendmail emerge sendmail # This finally worked According to peitolm "sendmail provides virtual/mta, but if you have mbox in USE, then it pulls in procmail. which requires virtual/mta but at the dependancy checking, I think it doesn't realise sendmail will provide it" I haven't put mbox into the USE, so I'm assuming this is a default. This is the third machine I've had this happen on.
*** This bug has been marked as a duplicate of 1343 ***