(Stolen from Michael's signature)

-----Original Message-----
From: mailop [] On Behalf Of Gilles Chehade
Sent: Wednesday, September 21, 2016 6:02 AM
Subject: [mailop] anyone from Microsoft around ?


I'm lead developer of an opensource MTA called OpenSMTPD.

We run a mailing-list for our project with very low volume, exchanging a few 
thousands messages each month to around 400 people, very few of them being 
hosted at big hosts, all of them having subscribed voluntarily.

In addition to this, we use a few mailboxes at major hosts and send some mails 
every now and then to ensure we didn't break the smtp engine, this usually 
amounts to less than 10 mails / month per big ISP.

All of these mails respect all best-practices that we know of:

- they are sent from long existing domains;
- they are rate-limited despite low volume;
- they are DKIM/DomainKeys signed;
- DNS is properly configured with valid DNS/rDNS for the one IP address;
- IP address is the same as the MX accepting mail for the domain;
- SPF is properly declared, so is DMARC;

I checked senderscore and we actually send so few volume we don't even have a 
reputation visible there.

I've started receiving complaints about users receiving their messages in the 
spambox only at microsoft-hosted domains. No problem for gmail, yahoo, orange 
and others, just microsoft.

I did some testing, and it turns out that even the simplest mails will end up 
spamboxed no matter what machine or domain I use to send. The reason advertised 
in the Microsoft appended domains may vary if I turn off DKim, strip some or 
all Received lines, but basically no matter my configuration, mails will not be 

I'm wondering if there is something wrong in the format of our headers and if 
this could affect users of our software disregarding the kind of mails they 
send with it. I've been tackling this for hours before contacting the mailing 

Any help would be greatly appreciated, I'm running out of ideas :-)

Gilles Chehade                                          @poolpOrg

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