Why do gmail emails end up in spam?

Question

gmail

DMARC SPF DCIM - PASS
ipv6 added to SPF
mail-tester.com - 10/10

Delivered-To: [email protected]
Received: by 2002:adf:aee4:0:0:0:0:0 with SMTP id y91-v6csp482264wrc;
        Thu, 7 Jun 2018 23:02:25 -0700 (PDT)
X-Google-Smtp-Source: ADUXVKJzQBXTnXax7X63prM332sK2HpibC2YOomkWOD3cIklRnO2996VZcLVje4gA+92LyifHpHU
X-Received: by 2002:a2e:4942:: with SMTP id b2-v6mr556835ljd.138.1528437745051;
        Thu, 07 Jun 2018 23:02:25 -0700 (PDT)
ARC-Seal: i=1; a=rsa-sha256; t=1528437745; cv=none;
        d=google.com; s=arc-20160816;
        b=AtJLf3fJmer9qD0hjk1iD6SDVxzh4Z4o+Mx1rct4QDg+8br6pvEqHGcv16rd9nyypI
         5j3ULepR0kyonRzbdb/QkkbGAlcBnQnkVpGvyL0sroLGFDE0ZW5/OYiQAPEyly9179Sy
         A/iM+WomJZLMUOri8glxLb5GxyKQ1+OthrOlixTMzBESzc7hknLRHRR5Wct91yIH5OG5
         yHJjXMXDO54+9AiEQgB84NXYTPiqspzFzEDaMiNlosRP4bfhsYz9JgL6AyVHj/2ozx2X
         eZet0jgnkj4XGkInvJkR2cM9SKrrad5/kdn2okvbiE3F3rMDZM3fBY6KcqxMSyO9NQnR
         D+Yg==
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816;
        h=mime-version:to:from:subject:date:message-id:dkim-signature
         :arc-authentication-results;
        bh=s0ReK9FPFRSU0cOvNfqkpS/WyMlcvDEUL4Jjc7xxWNI=;
        b=nvQMGdwBGhpbyHtcTxQwL0COo2CDn/dz/bVwVHZpbDR5KpCG3+kmajM27wn0i5OTY5
         ULyChw3/ksGyluVgZ+FQxWyL49GHR7gvm0agsRdjzRBF3bMAt0hAJ8IygwmvhKc9eX/d
         VENTY3i3cRLC/RDtxWK4N1PzjLauY4dSpb+UEufciHaRMmEe/OGnW+2aukOOHLRoTD1G
         WM+Rlu8lf+61+jXpeFqo4Rxqrcu6kXxaKekuCyc80myuhs+ifcLXmp7o5cpIEKWwxENo
         9ryl5rhECXXZ30EXR8UKL7ABJyCu3/U24JptfryMdndiwsXFWoMyT6UajiaM7O/BWSto
         dSsQ==
ARC-Authentication-Results: i=1; mx.google.com;
       dkim=pass [email protected] header.s=dkim header.b=h7sdsNQb;
       spf=pass (google.com: domain of [email protected] designates 2a00:f940:2:1:2::1d20 as permitted sender) [email protected];
       dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=diso.ru
Return-Path: <[email protected]>
Received: from diso.ru ([2a00:f940:2:1:2::1d20])
        by mx.google.com with ESMTPS id v6-v6si8275199ljh.292.2018.06.07.23.02.24
        for <[email protected]>
        (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
        Thu, 07 Jun 2018 23:02:25 -0700 (PDT)
Received-SPF: pass (google.com: domain of [email protected] designates 2a00:f940:2:1:2::1d20 as permitted sender) client-ip=2a00:f940:2:1:2::1d20;
Authentication-Results: mx.google.com;
       dkim=pass [email protected] header.s=dkim header.b=h7sdsNQb;
       spf=pass (google.com: domain of [email protected] designates 2a00:f940:2:1:2::1d20 as permitted sender) [email protected];
       dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=diso.ru
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=diso.ru;
  s=dkim; h=Content-Type:MIME-Version:To:From:Subject:Date:Message-ID;
  bh=s0ReK9FPFRSU0cOvNfqkpS/WyMlcvDEUL4Jjc7xxWNI=; b=h7sdsNQb5R3BMpGhgPlOcekXLx
  vd5xTroEJ98OqNFgHYRzcXTtuJI20M2mBhGiegs1YFOX7tgd+qBkRX+89SvagFXDA8r9fIPcpZJ2L
  yPUmml0iSjBhDqCgfpMrJAxN6r/r6SVi2WsEZLslzpdRv36CqK0G0LHsV2FLf3v6GsX4=;
Received: from diso.ru ([134.0.116.92] helo=api.diso.ru)
  by diso.ru with esmtpa (Exim 4.86_2)
  (envelope-from <[email protected]>)
  id 1fRATg-0003kW-Fe
  for [email protected]; Fri, 08 Jun 2018 07:02:24 +0100
Message-ID: <[email protected]>
Date: Fri, 08 Jun 2018 06:02:24 +0000
Subject: =?UTF-8?Q?=D0=9D=D0=BE=D0=B2=D1=8B=D0=B9_=D0=BF=D0=B0?=
 =?UTF-8?Q?=D1=80=D0=BE=D0=BB=D1=8C?=
From: "DISO.RU" <[email protected]>
To: [email protected]
MIME-Version: 1.0
Content-Type: multipart/alternative;
 boundary="_=_swift_1528437744_bbd583d7448c5e96175cac6b9fd790ed_=_"


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Content-Transfer-Encoding: quoted-printable

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1fu

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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org=
/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns=3D"http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3DUT=
F-8" />
    <title></title>
    </head>
<body>
       =20
<div>
    <p>=D0=97=D0=B4=D1=80=D0=B0=D0=B2=D1=81=D1=82=D0=B2=
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</div>
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</html>


--_=_swift_1528437744_bbd583d7448c5e96175cac6b9fd790ed_=_--

Answer 1

>host -tAAAA diso.ru
diso.ru has no AAAA record

PTR resolves to a name, and a name does not resolve back to the same IP, IPv6 addresses usually require strict FCrDNS passing.
But there can be many reasons, for example, a bad reputation for a domain.