gentoo-overlay/mail-client/nail/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <herd>net-mail</herd>

  <use>
    <flag name='net'>
      Enable support for network protocols (POP, IMAP and SMTP). If you
      only need to send mail with the local Transport Agent, disabling
      this will get you support for only /usr/sbin/sendmail call.
    </flag>

    <flag name='ssl'>
      If network is enabled, this adds support for S/MIME and
      SSL/TLS-powered protocols through <pkg>dev-libs/openssl</pkg>.
    </flag>

    <flag name='kerberos'>
      If network is enabled, this adds support for GSSAPI login on IMAP
      through <pkg>virtual/kerberos</pkg>.
    </flag>
  </use>
</pkgmetadata>