Okay the "FROM:" and "TO:" lines are not really send, it is just debug output from 
Mailer.smtp_send(). And the case insensitivity explains why we get two comparisons,
"From:" and "FROM:" are found as two different keys, but they both match both values.

Reading the tests, testMultipartRFC822 was missing the FROM:
and testUnknownUser just had the wrong line in there with the display. This is easy to fix.

changeset:   4974:65a76a7f3e01
user:        Bernhard Reiter <bernhard@intevation.de>
date:        Wed Mar 11 21:47:41 2015 +0100
files:       test/test_mailgw.py
description:
issue2550877 partly fixed.
  * Added docstring hint to _get_mail().
  * Fixed FROM: lines in testMultipartRFC822() and testUnknownUser()

So we are down to 3 tests failing because of to: missmatches, have to find out
what the correct behaviour is, here.