Why does Outlook (IMAP) periodically download a blank email?

Question

Email

Good afternoon!
Most email clients in the organization are Outlook. IMAP server - dovecot 2.2.21. I observe a very rare and therefore almost imperceptible problem - Outlook loads attachments or the body of the letter, clogged with zeros. Displayed as an empty letter. Opening this email through the web client or reloading the email downloads it successfully. Letter example:

spoiler

Received: from mail.ru ([x.x.x.x])
  by mail.bte-atm.ru with esmtps (TLSv1:ECDHE-RSA-AES256-SHA:256)
  (Exim 4.86_2 (FreeBSD))
  (envelope-from <[email protected]>)
  id 1bDoQB-00004r-5k
  for [email protected]; Fri, 17 Jun 2016 10:42:37 +0300
Received: from SSMLHTS.ru.ru (x.x.x.x) by mail.ru.ru
 (x.x.x.x) with Microsoft SMTP Server (TLS) id 14.3.301.0; Fri, 17 Jun
 2016 10:42:31 +0300
Received: from SBORMBX1.ru.ru ([x::x:x:x:x]) by
 SSMLHTS.ru.ru ([::1]) with mapi id 14.03.0294.000; Fri, 17 Jun 2016
 10:42:29 +0300
Return-Path: <[email protected]>
From: "Ka I" <[email protected]>
To: =?UTF-8?B?0K3Qu9GM0LLQuNGA0LAg0KDQsNC90LPRg9C70L7QstCw?= <[email protected]>,
  "KK" <[email protected]>
Cc: "EII" <[email protected]>,
  "AO" <[email protected]>
References: <[email protected]> <[email protected]> <00a701d1c79f$e49a73f0$ad[email protected]> <[email protected]> <[email protected]> <[email protected]> <[email protected]>
In-Reply-To: <[email protected]>
Subject: =?UTF-8?Q?RE:_1606080203_=D0=95=D0=BA=D0=B1?=
Date: Fri, 17 Jun 2016 10:42:28 +0300
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: multipart/alternative;
  boundary="----=_NextPart_000_00DC_01D1C887.4700CCF0"
X-Mailer: Microsoft Outlook 14.0
Thread-Index: AQFOf19mMBikN9jSPjP2CtAQZ9dizwJ4dwCmAW4ntrkBudRoJgF/eLx9AdB6AE4CB098ZwKX/oEm

This is a multipart message in MIME format.

------=_NextPart_000_00DC_01D1C887.4700CCF0
Content-Type: text/plain;
  charset="UTF-8"
Content-Transfer-Encoding: 7bit


------=_NextPart_000_00DC_01D1C887.4700CCF0
Content-Type: text/html;
  boundary="_005_BD775A3FD5D1B143AAC6DAAA5D208F9001485259SBORMBX1ru_";
  charset="UTF-8";
  type="multipart/alternative"
Content-Transfer-Encoding: quoted-printable


=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=
=00=00=00=00=00=00=00=00=00=00=00=00=00
------=_NextPart_000_00DC_01D1C887.4700CCF0--

The logs on the server side do not contain errors. Logging from the client side could help, but it's too rare a problem to be caught by enabling IMAP logging for all employees.
Maybe a familiar situation for someone? Can you suggest an adequate way to diagnose this problem?