mail-mta/msmtp already had an mta USE flag and I'm going to introduce one for bug #289973. The problem is that while virtual/mta allows USE dependencies, a number of packages don't allow that in any of their ebuilds because of EAPI restrictions: mail-mta/courier mail-mta/esmtp mail-mta/exim-4.71 (later versions are fine) mail-mta/mini-qmail mail-mta/nbsmtp mail-mta/netqmail mail-mta/nullmailer mail-mta/qmail-ldap
I am pretty sure we have a new-style virtual/mta these days and it's even maintained. commit bb317bae6e2957383a9d455f46d6a2ae591d0a6b Author: Mikle Kolyada <zlogene@gentoo.org> Date: Wed Mar 27 20:01:31 2019 +0300 virtual/mta: add myself as maintainer Signed-off-by: Mikle Kolyada <zlogene@gentoo.org> Package-Manager: Portage-2.3.62, Repoman-2.3.11