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

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.

<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 also encoded to mime. This is the code that does the splitting https://api.drupal.org/api/drupal/core!lib!Drupal!Component!Utility!Unic...

Locally my MailHog is getting all the email correctly. These are the headers:

Content-Transfer-Encoding: 8Bit
Content-Type: text/plain; charset=UTF-8; format=flowed; delsp=yes
From: =?UTF-8?B?RWVzdGkgS2F1YmFuZHVzLVTDtsO2c3R1c2tvZGEgPGtyaXN0by52aW50QHd1bmQ=?= =?UTF-8?B?ZXIuaW8+?=
MIME-Version: 1.0
Message-ID: zLXStg0HJsdjm3puvH4Lr4FVTKNNKLnJ0APSbxCRP7c=@mailhog.example
Received: from localhost by mailhog.example (MailHog)
          id zLXStg0HJsdjm3puvH4Lr4FVTKNNKLnJ0APSbxCRP7c=@mailhog.example; Fri, 24 Mar 2017 11:44:16 +0200
Return-Path: <kristo.vint@wunder.io>
Sender: kristo.vint@wunder.io
Subject: =?UTF-8?B?W0vDvHNpIGxpc2FdIHRlc3QxMA==?=
To: hannes.kirsman@wunder.io
X-Mailer: Drupal

Tere!

What do you think is going on here?