What is returnpath.net? They sent an automatic complaint allegedly for spam, did anyone come across it?

Question

Email

I have a project in which notifications about events, news, etc. are sent to users. About 250-300 thousand letters a week to understand the scale. All letters are signed with DKIM, SPF, DMARC are configured. All recipients, without exception, registered with us themselves and entered their mail themselves, there is no spam. Until recently, they sent mail through a third-party service, recently I had to plunge into the dark world of mass mailings.
Today the hoster received a complaint (the first ever) about the spam we send out. I asked for the original letter of complaint and found:
1) the complaint was created automatically
2) From contains the address [email protected] (despite the fact that there is no feedback.mail.ru in DNS, neither A nor MX)

% host -t any feedback.mail.ru
Host feedback.mail.ru not found: 3(NXDOMAIN)

3) sent from some returnpath.net

Received: from mrb-fbl.returnpath.net (mrb-fbl.returnpath.net [66.45.29.179])
        by mxfront7g.mail.yandex.net (nwsmtp/Yandex) with ESMTP id cKnHF8UWoc-hoJKUYjs;
        Thu,  3 Dec 2015 08:43:50 +0300
Authentication-Results: mxfront7g.mail.yandex.net; spf=pass (mxfront7g.mail.yandex.net: domain of senderscore.net designates 66.45.29.179 as permitted sender) [email protected]
Message-Id: <[email protected]>

4) the complaint is accompanied by the original letter with headers, which are complained about, where it is indicated that the letter was supposedly sent to the mailbox [email protected] , I checked, this email does not exist at all.

Received: from *** (unknown [10.71.112.14]) by
  outmx.***.ru (Postfix) with ESMTP id 3pB93F6yD8z2sgL for
  <[email protected]>; Thu,  3 Dec 2015 11:15:53
  +0300 (MSK)

in fact, instead of [email protected] there should be another mailbox ***@yandex.ru , I found it in the logs.
5) DKIM signature in the attached letter, of course, does not fit.
So, I have a question, what is the returnpath.net office that sends fictitious complaints? Well, apparently, your experience, how to protect yourself from this in the future.

Answer 1

Did you find a solution to the problem? also wondering how to solve this quest