Zend Framework 2(MimePart)用“=?utf-8?Q?”编码的消息头

If I send Mail from my local server in message source I see it:

Content-Type: text/html
Content-Transfer-Encoding: 8bit

But, if I send Mail from another server, message source looks like this:

Content-Type: =?utf-8?Q?text/html?=
Content-Transfer-Encoding: =?utf-8?Q?8bit?=

My ZF2 application is identical in both servers. I send mail using zf2 SMTP transport class.
Local server running under Ubuntu 13.10 + Apache, another server under Ubuntu 13.04 + Nginx

Why message sources are not identical?