router = lookuphost, transport = remote_smtp
  host gmail-smtp-in.l.google.com      [72.14.247.27]   MX=5
  host alt1.gmail-smtp-in.l.google.com [72.14.205.27]   MX=10
  host alt2.gmail-smtp-in.l.google.com [64.233.183.27]  MX=10
  host alt2.gmail-smtp-in.l.google.com [64.233.183.114] MX=10
  host gsmtp163.google.com             [64.233.163.27]  MX=50