First of all this error occurs.
===
Unfortunately, messages from xx.xx.xxx.xxx weren't sent.
Please contact your Internet service provider since part of their network is on our block list.

And we received such a message too.
===
Not qualified for mitigation
xx.xx.xxx.xxx
Our investigation has determined that the above IP(s) do not qualify for mitigation.

Here is a header from a test mail today. We tried to contact the Microsoft support, but never got an answer, why we on blacklist are. We are not on DNSBL, and have green lights on hundreds of other blacklist monitors.
========

Return-Path: <>
Delivered-To: <                @mailbox.hu>
Received: from mail.mailbox.hu ([127.0.0.1])
    by iredmail01.localdomain (Dovecot) with LMTP id YHYZNMx94lZaHQAALC3GDA
    for <                @mailbox.hu>; Fri, 11 Mar 2016 09:12:07 +0100
Received: by mail.mailbox.hu (Postfix)
    id EFF57403615; Fri, 11 Mar 2016 09:12:07 +0100 (CET)
Date: Fri, 11 Mar 2016 09:12:07 +0100 (CET)
From: MAILER-DAEMON@mailbox.hu (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To:                 @mailbox.hu
Auto-Submitted: auto-replied
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
    boundary="604FA403609.1457683927/mail.mailbox.hu"
Content-Transfer-Encoding: 8bit
Message-Id: <20160311081207.EFF57403615@mail.mailbox.hu>

This is a MIME-encapsulated message.

--604FA403609.1457683927/mail.mailbox.hu
Content-Description: Notification
Content-Type: text/plain; charset=us-ascii

This is the mail system at host mail.mailbox.hu.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<ugyfelszolgalat1234@hotmail.com>: host mx1.hotmail.com[65.55.37.88] said: 550
    SC-001 (COL004-MC2F18) Unfortunately, messages from 91.82.220.225 weren't
    sent. Please contact your Internet service provider since part of their
    network is on our block list. You can also refer your provider to
    http://mail.live.com/mail/troubleshooting.aspx#errors. (in reply to MAIL
    FROM command)

--604FA403609.1457683927/mail.mailbox.hu
Content-Description: Delivery report
Content-Type: message/delivery-status

Reporting-MTA: dns; mail.mailbox.hu
X-Postfix-Queue-ID: 604FA403609
X-Postfix-Sender: rfc822;                 @mailbox.hu
Arrival-Date: Fri, 11 Mar 2016 09:12:06 +0100 (CET)

Final-Recipient: rfc822; ugyfelszolgalat1234@hotmail.com
Original-Recipient: rfc822;ugyfelszolgalat1234@hotmail.com
Action: failed
Status: 5.0.0
Remote-MTA: dns; mx1.hotmail.com
Diagnostic-Code: smtp; 550 SC-001 (COL004-MC2F18) Unfortunately, messages from
    91.82.220.225 weren't sent. Please contact your Internet service provider
    since part of their network is on our block list. You can also refer your
    provider to http://mail.live.com/mail/troubleshooting.aspx#errors.

--604FA403609.1457683927/mail.mailbox.hu
Content-Description: Undelivered Message
Content-Type: message/rfc822
Content-Transfer-Encoding: 8bit

Return-Path: <                @mailbox.hu>
Received: from localhost (amavis1 [10.0.30.22])
    by mail.mailbox.hu (Postfix) with ESMTP id 604FA403609
    for <ugyfelszolgalat1234@hotmail.com>; Fri, 11 Mar 2016 09:12:06 +0100 (CET)
X-Virus-Scanned: Debian amavisd-new at amavis1.mailbox.hu
Received: from mail.mailbox.hu ([10.0.30.21])
    by localhost (mailbox.hu [10.0.30.22]) (amavisd-new, port 10024)
    with ESMTP id g23m7es6RxPo for <ugyfelszolgalat1234@hotmail.com>;
    Fri, 11 Mar 2016 09:11:53 +0100 (CET)
Received: from mailbox.hu (localhost [127.0.0.1])
    by mail.mailbox.hu (Postfix) with ESMTPSA id A8754403612
    for <ugyfelszolgalat1234@hotmail.com>; Fri, 11 Mar 2016 09:12:02 +0100 (CET)
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=mailbox.hu;
    s=mbxselector; t=1457683923;
    bh=qxwHqKv/Tpjf9256mKPSwDe5OY0uVvvh8XCM7N/lFes=;
    h=Date:From:To:Subject:From;
    b=UB/PNEcIyKDEKTlFAoYArUhM2sBTIdSrl02MEsy0MB/2u82d84xwx43jh/W9AcYnL
     VI+v1BmoO066XL4dS36wUNYtAps+QL+iTZ4rbOPLg4FUYGAeLMrUQQ5sywTd8OHoD7
     INJZ2vMTMbREzp8TowGhK58mDoOLsJradFp5gB6o=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8;
format=flowed
Content-Transfer-Encoding: 8bit
Date: Fri, 11 Mar 2016 09:12:02 +0100
From: =?UTF-8?= <                @mailbox.hu>
To: ugyfelszolgalat1234@hotmail.com
Subject: Microsoft Cloud Test
Message-ID: <3dc9a8302622d5f5de3020393a0d9195@mailbox.hu>
X-Sender:                 @mailbox.hu

Oh, we did it many times. The "policy reasons" error explanation is so deep mysterious what we can not interpret. What if dear Microsoft would give us more explicited error message? Be it private, we would happy and could tune our system. That is why I listed all of the security tasks what we did for a secure operation.