Funtoo stages have mail-mta/msmtp as default mta package. This package is no longer active in development (if not dead). Personally, i use postfix on all of my VE, it is not hard to setup. Proposal is to replace msmtp with postfix.
Plan of replacement:
1.fork virtual/mta and set postfix as default. there will be automatic unmerge of msmtp
2. provide basic postfix setup, wiki page.
Known issues and improvements with postfix:
1. setting hostname in main.cf
2. unix sockets (this is fixed in forked ebuild)
3. automatically create aliases db (done in forked ebuild)
Remove _pre releases of postfix and stick with stable versions, 2.9.3, and stable-lts 2.8.11