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 inboxed.

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 list.

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

Gilles Chehade

https://www.poolp.org                                          @poolpOrg

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