Why are letters duplicated to another addressee?

Question

Email

Good day!
There are two employees. One has mail of the form g *** [email protected], the second [email protected]***m.ru (domain mail from Yandex)
Thunderbird receives mail from the first employee, collecting it from both mailboxes.
In the mail for [email protected]**m.ru (i.e. in Yandex), the collection of mail from g***[email protected] is configured.
Letters sent from the mail of the first employee (with two accounts) end up in the inbox of the second (with one [email protected]***m.ru account), although it was not listed at all in the recipients. Here are the official headers of such a letter:

Received: from mxback6o.mail.yandex.net ([127.0.0.1])
  by mxback6o.mail.yandex.net with LMTP id Y3ENXS9s
  for <[email protected]***m.ru>; Thu, 4 Feb 2016 08:21:14 +0300
Received: from rpop2o.mail.yandex.net (rpop2o.mail.yandex.net [37.140.190.59])
  by mxback6o.mail.yandex.net (nwsmtp/Yandex) with ESMTP id Jdw9gFegGk-LE3mcq1i;
  Thu,  4 Feb 2016 08:21:14 +0300
X-Yandex-Front: mxback6o.mail.yandex.net
X-Yandex-TimeMark: 1454563274
X-Yandex-Spam: 1
X-yandex-pop-server: imap.mail.ru
X-yandex-rpop-id: 24000135153
X-yandex-rpop-info: g***[email protected]
Received: from g***[email protected] ([185.5.137.105])
  by mail.yandex.ru with IMAP id CGDfKp0m3qMx
  for [email protected]; Thu, 4 Feb 2016 08:21:14 +0300
Delivered-To: g***[email protected]
Received: from 178.129.251.243:51761
  by 217.69.139.90:993 with IMAP
  Thu, 04 Feb 2016 05:04:29 +0000 (GMT)
Message-ID: <[email protected]>
Date: Thu, 04 Feb 2016 10:02:12 +0500
From: =?UTF-8?B?0JPQsNGA0LjRhNGD0LvQu9C40L0g0KDRg9GB0LvQsNC9?=
 <g***[email protected]>
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.28) Gecko/20120313 Thunderbird/3.1.20
MIME-Version: 1.0
To: [email protected]
Content-Type: multipart/mixed;
 boundary="------------040200080204030206040108"
Return-Path: [email protected]***m.ru
X-Yandex-Forward: ee038adf0f41497b3da5bf580faa3fe4

This is a multi-part message in MIME format.
--------------040200080204030206040108
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

It started about a week ago, before this it was not observed. Where is the dog buried?

Answer 1

This message was placed via IMAP by the mail client in "Sent Items" (or another folder according to the client's settings) by the user g***[email protected], then it was picked up via IMAP by Yandex and placed in the folder by the user [email protected]*** m.ru.
Make sure that the folders for sent letters are correctly configured for employees (in Thunderbird in the account properties "Copies and folders", "automatically place a copy in") and mail in mail.ru goes exactly to "Sent". Check that there are no filters in the Yandex interface and in Thunderbird for an employee with a mailbox on Yandex. If everything is set up correctly and there are no filters, try contacting Yandex support, that the IMAP mail collector from mail.ru puts letters from those sent into the inbox.

Answer 2

I can only guess.
1. A rule (filter) could have been created on your Yandex mail that moves such letters to the Inbox.
2. Or the collection of letters on Yandex is carried out using the POP protocol and the Inbox folder is specified for collection.
3. Thunderbird is mentioned in the comments to my answer.