Phillip,
[EDIT: Posts crossing - this took a deal of drafting]
One understands all the business needs you mention; what one is seeking to do is identify the key stumbling blocks which are giving rise to the issues you are encountering. As you state, this has all worked fine for years, now it does not ... so what has changed?
Previously all *.plus.com emails were DKIM signed with a DMARC policy of "we do not direct what YOU should decide to do with a DKIM failure" in other words, we do not tell you how to run your email service. In other words YOU the receiver apply your own policy. These form two of the key measures for verifying the legitimacy of an email in an attempt to reduce spam. That coupled with content inspection is a fairly robust approach. Under that framework all has worked well since the days of the first round of bully-boy tactics from Gmail.
Recently Microsoft made a unilateral draconian decision that ALL sub-domains of a root domain are to be treated as a single source of email and where the volume of emails from that source exceeds 5,000 per day, the root domain will be designated a bulk sender and subject to the imposition of additional rules. In short, Microsoft has started to tell the world how everyone else should run their email services. Where a root domain is deemed a bulk sender, the following must be implemented...
- SPF
- DKIM
- DMARC
In general the requirements have been either of 1 or 2 with 3 being optional, now all three are mandatory for a bulk sender. Consequently emails sent to any Microsoft service were being bounced straight back to Plusnet sender.
Currently reluctantly accepting that *.plus.com is deemed a bulk sender, SPF was added to each *.plus.com sub-domain; there after emails have been delivered to Microsoft mailboxes, where the were not upon the change being implemented.
You then ruck up with this very interesting fringe case: can send emails to individual Hotmail and Live mailboxes absolutely fine - SPF=PASS, DKIM=PASS, DMARC=PASS - but attempt to send one email to several of those same email addresses concurrently and it all falls apart - SPF=PASS, DKIM=FAIL, DMARC=PASS.
What is the key change? Multiple addresses, visible to the receiving service. Who knows what the receiving service will make of that, like everything else with Microsoft, it is guess work ... because if they gave the answers, such would be deemed helpful to the real villains to circumvent the anti-spam measures.
There are many reports of issues with hotmail email targets, one of which is potentially very interesting: where you have advised that sending to a single addressee succeeds but sending to several fail, is that EXACTLY the same email?
See Issues with mailing into Microsoft domains - Microsoft Q&A
We are facing the same problem: everything is OK, DKIM/SPF/DMARC but that same error comes when the subject line contains some iso8859-1 characters like this one :
Subject: Mise à jour
Hotmail and others break on the dkim validation because of it.
Subject lines with UTF-8 characters seem ok, quoted printable ok too.
You may try to simulate the error with another pure-ascii subject line to validate this.
Microsoft have not offered any answers...
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
