I managet to hijack the email on molal MTA:
From MAILER-DAEMON Thu Oct 29 11:36:02 2015
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Subject: [GitLab Mattermost] Project Team Notifications for Oct 29, 2015
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Date: Thu, 29 Oct 2015 11:36:01 +0100
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[...]
<p>CHANNEL: piotr<br>krzysztof- 10:36 GMT, Oct 29<br><pre style="text-align:left;font-family: 'Lato', sans-serif; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">[4] test mattermost ążćńźłóęśń ĄŻŹĆŃŁÓŚĘ [2] test mattermost ążćńźłóęśń ĄŻŹĆŃŁÓŚĘ</pre></p>
[...]1) So in terminal or notepad++ I can see polish native characters (as intended in UTF8). As I suspected there is no encoding in mail headers.
2) The "From:" fiels is empty
K.