Почта отправляется с сервера, плюс с яндекса. В данном примере удалены DKIM DMARС Яндекса, ибо там то все работает.
Все записи настроены и работают
Проверяем на случай рукопожости.
https://mxtoolbox.com/domain/dairub.ru/
Однако, даже ошибок нет, тупо нет проверки ни SPF ни DMARC
Received: from mxfront9g.mail.yandex.net ([127.0.0.1])
by mxfront9g.mail.yandex.net with LMTP id gEJINozk
for <viktor@7877796.ru>; Thu, 22 Nov 2018 21:47:18 +0300
Received: from ks05.ru (ks05.ru [95.216.36.253])
by mxfront9g.mail.yandex.net (nwsmtp/Yandex) with ESMTPS id xF9meubYN7-lHEGELsv;
Thu, 22 Nov 2018 21:47:17 +0300
(using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits))
(Client certificate not present)
Return-Path: web10@ks05.ru
X-Yandex-Front: mxfront9g.mail.yandex.net
X-Yandex-TimeMark: 1542912437
Authentication-Results: mxfront9g.mail.yandex.net; dkim=pass header.i=@dairub.ru
X-Yandex-Spam: 1
Received: from localhost (localhost.localdomain [127.0.0.1])
by ks05.ru (Postfix) with ESMTP id B0B1CF00EFB
for <viktor@7877796.ru>; Thu, 22 Nov 2018 21:47:16 +0300 (MSK)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=dairub.ru; h=date
:date:message-id:x-mailer:from:from:subject:subject; s=default;
t=1542912434; x=1544726835; bh=frcCV1k9oG9oKj3dpUqdJg1PxRT2RSN/
XKdLCPjaYaY=; b=Vn3IjcFOHx3TEdyoKs9XXYcRerXbDozQoXTzMoLAb6qgHbai
6gUypyYiiYFOWF6g/Awhhjl/6wnmp+GxyTm+1nSdAbui1KLC8sJTfeNgTi0XMijP
pecC/LF86LMrl+rD6DF42b4IzQ0kINjIdDmhIEM5LzyxHMDRHxmMKw5d+/Y=
X-Virus-Scanned: Debian amavisd-new at ks05.ru
Received: from ks05.ru ([127.0.0.1])
by localhost (ks05.ru [127.0.0.1]) (amavisd-new, port 10024)
with ESMTP id ZTugkeoq1C8O for <viktor@7877796.ru>;
Thu, 22 Nov 2018 21:47:14 +0300 (MSK)
Received: by ks05.ru (Postfix, from userid 5013)
id E5F33F00F18; Thu, 22 Nov 2018 21:47:14 +0300 (MSK)
To: viktor@7877796.ru
Subject: the subject
X-PHP-Originating-Script: 0:mail.php
From: info@dairub.ru
X-Mailer: PHP/5.6.38-2+0~20181015120829.6+stretch~1.gbp567807
Message-Id: <20181122184714.E5F33F00F18@ks05.ru>
Date: Thu, 22 Nov 2018 21:47:14 +0300 (MSK)
X-Yandex-Forward: 107bce4f08bc9bf13e66c15aa5ddb76a
Может что-то в postfix ?
dairub.ru/main.cf.txt
dairub.ru/master.cf.txt
dairub.ru/phpinfo.php 
Средний
