Mail subject shows utf-8 encoding

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Concrete5.7.5.6
PHP 5.6.30
SMTP: yes

I have a custom block that uses the sendmail to send a notification but the subject line shows the utf-8 encoding. The same encoding you see in the logs.

=?UTF-8?Q?Brochure=20aanvraag, =20gebouw=2002=20-=20Appartement=20A105?=

I'm trying to figure out at what point this fails. (Did not make the block myself.)
$mh->setSubject($this->parseString($this->emailSubject));
$mh->setBodyHTML($this->parseString($this->emailContent));
@$mh->sendMail();

parseString returns the string with the vars replaced, if I leave it out it just shows the same but without the replacement:
=?UTF-8?Q?Brochure=20aanvraag, =20gebouw=20{{=20page::building=20}}=20-=20?= {{ page::name }}

Strange part: This only happens on the live server, mails send from localhost version works as expected (clean email subject). - Local php version: 5.6.10

The default formblock doesn't have this problem.

c5dragon