Use message sender and from addresses for SMTP reverse path if valid.

This stops the primary account being leaked when using an alt sender
address as long as the server does not re-write it. See issue #18.
6 jobs for wip/18-primary-email-leak in 13 minutes and 40 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #56185
deb-package

00:06:34

passed #57683
fedora

00:05:53

passed #57684
flatpack-package

00:06:13

passed #56184
ubuntu

00:07:24

failed #56183
fedora

02:21:04

failed #56186
flatpack-package

00:00:17