Why do emails from some senders within a domain end up in spam?

Question

Mail server

Colleagues, give fresh ideas. In an Exchange 2010 company, there are several users whose emails end up in spam during internal correspondence. Employee A writes to employee B and ends up in spam. Moreover, if employee A writes to someone else, then everything is OK. The rules and blacklists of the users have been checked, neither Spamassassin nor NOD have anything to do with this, the subject of the letter does not change. Logs on the server are absolutely normal.
Header of one of the letters:
Received: from ***************y.com ([1***************30]) by
** ***********y.com ([1***************6]) with mapi id 14.03.0352.000; Tue,
28 Nov 2017 11:22:19 +0300
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: =?koi8-r?B?4sHEzcHF1yDhytMg5NbBzsfB0s/Xyd4=?=
The To: = koi8-r Bed and 68HSwdPF1yDhztTPziD3zMHEyc3J0s / Xyd4 = =???? The
Subject: = koi8-r Bed and Rlc6IOvB08HUxczYzs8g0NLFxM / T1MHXzMXOydEg5M / XxdLFzs7P09TJIOjB =????
= Koi8-r Bed and zc3B1M / Xz8ogLyDv8CDr6e7n6fPl8PA = =????
Thread A-the Topic : = koi8-r Bed and 68HTwdTFzNjOzyDQ0sXEz9PUwdfMxc7J0SDkz9fF0sXOzs / T1Mkg6MHNzcHU =???? =
r-koi8 Bed and z9fPyiAvIO / wIOvp7ufp8 Xw8A == + =????
Thread A-the Index: AdNoG / b68b9j1QqhTd6sNHI0eGEHRAAAS5bgAAAJMkAAARhBsA == a
Date: Tue, 28 Nov 2017 11:22 :18 +0300
Message-ID: <[email protected]***************y.com>
References: <[email protected]*************** y.com>
<[email protected]***************y.com>
Accept-Language: ru-RU, en-US
Content-Language: ru-RU
MIME -Version: 1.0 X-MS -
Exchange -Organization-AuthSource: ****mail*******y.com X-MS-Exchange-Organization-AuthAs: Internal X-MS-Exchange-Organization-AuthMechanism: 04 X-Originating-IP: [192.168 .*****]

Answer 1

https://serverfault.com/questions/676256/internal-...
https://community.spiceworks.com/topic/1826628-int...
https://community.spiceworks.com/topic/375067-non -...
https://technet.microsoft.com/en-us/library/aa9968...
https://technet.microsoft.com/en-us/library/bb2016...
https://social. technet.microsoft.com/Forums/exchan...

Answer 2

Meanwhile, the problem is still not solved. We even tried disconnecting all client devices and logging in only through OWA. Those. the letter is thrown into spam NOT on the client device.