GMail rejects mail because long 'from' field is split by Drupal 8

Hi, FreeBSD community! Need help with some weird email problem I've come across first time in my 10-year developer career. I would apreciate any hint you could give! :)
We are hosting our Drupal 8 site on a FreeBSD 10.3 server. That server is managed by another company so we don't have access to root. They are also giving minimal answers so I'm trying to find more information about this issue before getting back to them.
Problem is that longer emails are sent back from Google because the 'from' field is split.

Code:
<hannes.kirsman@wunder.io>: host aspmx.l.google.com[64.233.163.26] said:
    550-5.7.1 [185.7.252.97      13] Messages with multiple addresses in From:
    550 5.7.1 header are not accepted. c63si781792lfe.242 - gsmtp (in reply to
    end of DATA command)

Final-Recipient: rfc822; hannes.kirsman@wunder.io
Original-Recipient: rfc822;hannes.kirsman@wunder.io
Action: failed
Status: 5.7.1
Remote-MTA: dns; aspmx.l.google.com
Diagnostic-Code: smtp; 550-5.7.1 [185.7.252.97      13] Messages with multiple
    addresses in From: 550 5.7.1 header are not accepted. c63si781792lfe.242 -
    gsmtp


---------- Forwarded message ----------
From: "Eesti Kaubandus-Tööstuskoda <kristo.vint@wund"@atria.elkdata.ee, er.io>@atria.elkdata.ee
To: hannes.kirsman@wunder.io
Cc:
Bcc:
Date: Fri, 24 Mar 2017 09:12:19 +0200 (EET)
Subject: [Küsi lisa] test5
Hannes Kirsman (not verified) (hkirsman@gmail.com) sent a message using the
contact form at


From different documentations I see this is how it should suppose to work. There's size limit and characters should be latin. In our case we have some special characters in the 'from' field so the headers are encoded to mime. This is the code that does the splitting https://api.drupal.org/api/drupal/core!lib!Drupal!Component!Utility!Unicode.php/function/Unicode::mimeHeaderEncode/8.2.x

What do you think is going on here?
 
Sorry for adding new message but I could not edit the first post. Something about spam filter...
I'm using MailHog locally and it get's the e-mails correctly. Here are the headers.
 
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