From address doesn't meet the authentication requirements defined for the sender

[#1]

Single emails are not acceptable from a business perspective and now they too are being rejected with blocked server messages.

Plain text emails make no difference at all to deliverability & for us never have,

[#2]

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@PhilipHeyes wrote:

Single emails are not acceptable from a business perspective 

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T&Cs

2. Your use of the services

2.1. You must:

2.1.3. use the services for personal use in the UK (so don't use the services to run your own business, but a couple of work emails or occasional home working are okay) and in accordance with our Acceptable Use Policy;

Business use of this service pushes the bulk sender volume threshold.

[#3]

A dozen emails a week is hardly onerous when set next to the sending of hundreds of thousands of SPAM messages via a single customers account.

[#4]

A dozen times how many addresses?

If the volume is so low, sending individually is not that much of a chore.

Reading the Mailgun blog Microsoft appears to be not disclosing the cause of this issue with the impacted parties.  Therefore one needs to consider other interim mitigation measures.

[#5]

@Townman re #795

My question to @PhilipHeyes was specifically about the blacklisting aspect of his post #681. Sending emails individually and/or as plain text is irrelevant if the avasout server is blacklisted.

If Plusnet can get their servers off block lists then we are back to the original DKIM failure issue and your suggestions may work for some people.

[#6]

Re #767 and #771

Regarding the ongoing DKIM failure saga, I emailed the mighty Mailgun about their promise “we’ll continue to share what we learn”, given that it’s now over 3 months since their article.

Apparently, they have nothing new to report, merely:

“Microsoft is still experiencing issues regarding this and the industry is communicating with them”

And

“Since the resolution depends entirely on Microsoft, we unfortunately cannot provide any estimates or insights on when they might address it.”

Not looking good is it!

[#7]

That's Microsoft for you! 😁

[#8]

@abitpedantic 

That is really useful cross-industry intel ... hopefully that will now speak to the many around here who incorrectly believe that this is an issue which is in the gift of any one mail provider to resolve.  These statements align closely with the advice offered around here months ago, dismissed by many, some a tad rudely.

Unfortunately providers are in limbo with only Microsoft to blame, until they disclose what the mail providers are doing wrong (if anything) to give rise to variable validation of the same credentials, depending on character sets and sending client, when addressed to multiple addressees.

[#9]

Precisely, @Townman 

[#10]

Another firm engaging with DKIM=Fail from Microsoft : 

https://www.postmastery.com/outlook-deliverability-a-silent-shift-with-serious-consequences/

[#11]

Another excellent piece of analysis, inferring questionable processing by Microsoft.

That said, I would expect that if the issue were related to DNS formatting issues, then DKIM would fail all of the time, not just when there are multiple addressees.

[#12]

I've not got all the details but I've just heard some positive news on this, be interested to see if anyone is still seeing problems as of now or whether it's been resolved.

[#13]

This afternoon we have seen a new rejection this time from the DWP the NDR contained :

Reporting-MTA: dns; avasout-peh-002 [212.159.14.18]
Received-From-MTA: dns; xxxxxlaptop [86.xx.xx.254]
Arrival-Date: Fri, 17 Oct 2025 14:35:06 +0100


Final-recipient: rfc822; contact.name@dwp.gov.uk
Diagnostic-Code: smtp; 550 #5.7.1 Your access to submit messages to this e-mail system has been rejected.

Last-attempt-Date: Fri, 17 Oct 2025 14:35:06 +0100


This is what resulted when replying to a one to one email sent from the DWP,
we are trying to assist a person living out of a night shelter into affordable housing.

[#14]

Phillip,

That is a very different issue worthy of a separate thread.  I have asked the mods if they can do some shuffling.

The ask was in the context of sending to multiple Microsoft addresses, not other services.

[#15]

Quite an ask !

I would loose the will to live faced with the task for divvying up 800+ posts
that range from the purely factual to the entirely fanciful.