Exim4 why doesn't the text/html header work?

Question

linux

Installed exim on ubuntu 15. Set it up to send via Yandex. Emails are sent via PHP. Letters arrive, but the header of the letter beats and as a result comes like this:
Header:
=?UTF-8?B?0KLQtdGB0YI=?=
Body:
--2c17a0012755fd1566317c109825420a
Content-Transfer-Encoding: 8bit
Content-type: text/html; charset=utf-8
Valid until: <br/>01/10/2016
What could be the reason?