Setting MX when sending with POSTFIX and why does SpamAssassin swear at DOS_HIGHBIT_HDRS_BODY?

Question

postfix

Good afternoon!
The task is to send mail from your POSTFIX (from the address [email protected] ), and receive it to Yandex mail at the address of the same name.
The DNS says the following:

example.com 	MX 	mx.yandex.ru
yamail-MY_KEY.example.com 	CNAME 	mail.yandex.ru

What to write in the postfix settings in this case? Difficulties arose with the following fields, while I wrote it like this:

mydomain = example.com
myhostname = yamail-MY_KEY.example.com
mydestination = localhost.localdomain, localhost

In particular, it's not clear, should myhostname contain a CNAME alias or something else?
......
The next snag came with SpamAssassin! Sending via phpMailer latest version.

SpamAssassin Score: 2.989
 0.7 MIME_HTML_ONLY         BODY: Message only has text/html MIME parts
 0.4 HTML_MIME_NO_HTML_TAG  HTML-only message, but there is no HTML tag
 3.8 DOS_HIGHBIT_HDRS_BODY  Headers need encoding and body is highbit

1. What is the reason for DOS_HIGHBIT_HDRS_BODY ?? It gives a very high score.
2. Where does HTML_MIME_NO_HTML_TAG come from if there are html tags in the message?

Return-Path: <[email protected]>
X-Original-To: [email protected]
Delivered-To: [email protected]
Received: from yamail-MY_KEY.example.com(example.com[123.123.123.123])
  by yen.roundsphere.com (Postfix) with ESMTP id 123
  for <[email protected]>; Thu, 16 Oct 2014 05:11:00 -0400 (EDT)
Received: by yamail-MY_KEY.example.com(Postfix, from userid 11)
  id 1323; Thu, 16 Oct 2014 13:11:58 +0400 (MSK)
Date: Thu, 16 Oct 2014 13:11:58 +0400
To: [email protected]
From: =?UTF-8?B?123==?= <[email protected]>
Reply-To: =?UTF-8?B?123==?= <[email protected]>
Subject: =?UTF-8?B?0J/RgNC40LLQtdGCIQ==?=
Message-ID: <[email protected]>
X-Priority: 3
X-Mailer: PHPMailer 5.2.9 (https://github.com/PHPMailer/PHPMailer/)
MIME-Version: 1.0
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 8bit

Как дела <b>друг?!</b><i>Все ок?</i>

Thanks in advance!!