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

[#1]

For us all email recipients are always in the To:  field.
No recipients are CC: or BCC fields.


Looking at the last six reject messages from MS they look like :

References: <E1uqvGQ-000AVP-6D@inmx-ptp-009.plus.net>
In-Reply-To: <E1uqvGQ-000AVP-6D@inmx-ptp-009.plus.net>
Subject: Thurrock and Brentwood Mind
Date: Tue, 26 Aug 2025 16:11:58 +0100
Message-ID: <008301dc169b$c5bd9430$5138bc90$@keyline.plus.com>
MIME-Version: 1.0
Content-Type: multipart/mixed;
    boundary="----=_NextPart_000_0084_01DC16A4.2781FC30"
X-Mailer: Microsoft Outlook 16.0
Thread-Index: AdwWm50OinZXCg8lQLiKnINTPsUC/A==
Content-Language: en-gb
X-CMAE-Envelope: MS4xfMQxBHqHYqIk24u9+t0TDSF/Efn0TO0cvIqMF4JYcgXQ2AikCc0EAz+h4NBRKxbT8384MCcSxruX3PD1Yn1s4DR5l9PpVOTW03EYe0GY4aei11xHXTH7
 LzG5cj+rp40C0s/YsB+8csFvvY4HFe6PQYnzyaSY3qUWmBtqyktRBqNoAOGg8GoUpW1T6CrBu643FQ==

[#2]

@PhilipHeyes 

Thank you.  Do not know if you have given straws here or I'm looking at red herrings.

I have asked of those who will know better.

[#3]

@Townman 

The email I reported on in #599 was sent to myself and bcc'd to the list. Since then I've sent another plain text email in the same way but containing 15 MS-related addresses (and a lot of others). Again, no failures.

It's been so long since I had a rejection that I'm afraid I've binned all the failure messages 🙂 

[#4]

@abitpedantic 

Are all your success experiences related to using webmail (as mentioned in #574) or using Outlook (as mentioned in #599)?

Your evidence leans towards the character encoding and DKIM reported issue ... but that would not explain why this issue is not being seen with singularly addressed emails.

It is a real puzzle!

[#5]

@Townman 

Until today my "success experiences" were achieved by using Plusnet webmail. Today, for the first time, I sent two emails from MS Outlook as plain text rather than HTML. All emails were sent To myself, Bcc'd to others.

I have 4 more lists of people to email if there are any other variations you would like me to try (except of course that I must use bcc).

[#6]

The biggest contribution which could be made here is  having an email with addressees which fails to… then resend the sent email … but not as HTML.

If that delivers success, we have a precise focus for escalation to / with Microsoft.

Thank you for exploring a well defined comparison.

[#7]

This looks like a different type of bounce back from Hotmail this morning:

Reporting-MTA: dns; avasout-peh-001 [212.159.14.17]
Received-From-MTA: dns; DESKTOP
Arrival-Date: Fri, 29 Aug 2025 09:29:07 +0100

Final-recipient: rfc822; @xxxxx@hotmail.co.uk
Diagnostic-Code: smtp; 550 5.7.1 Unfortunately, messages from [212.159.14.17] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [Name=Protocol Filter Agent][AGT=PFA][MxId=11BBC4D558CF653B] [AMS0EPF000001B2.eurprd05.prod.outlook.com 2025-08-29T08:29:08.044Z 08DDE26AB2E473A8]

Last-attempt-Date: Fri, 29 Aug 2025 09:29:08 +0100

Is the "Internet service provider on block list" something new?

Previous bounce backs have been, for me, smtp; 550 5.7.515 Access denied, sending domain xxxxx.PLUS.COM doesn't meet the required authentication level. The sender's domain in the 5322.From address doesn't meet the authentication requirements defined for the sender.

[#8]

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@ebforum wrote:

This looks like a different type of bounce back from Hotmail this morning:

Is the "Internet service provider on block list" something new?

---

It is - how can a legitimate MTA be on a blocklist - M$ really, really have lost the plot. 

[#9]

If this is a new aspect can someone here advise those in Plusnet who are involved in the investigation with MS?

[#10]

I would guess that at least one of the SU's has probably already seen this.

[#11]

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@jab1 wrote:

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@ebforum wrote:

Is the "Internet service provider on block list" something new?It is - how can a legitimate MTA be on a blocklist - M$ really, really have lost the plot. 

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@ebforum 

This is not new - I have seen this for years in respect of my Church's email service, which is AWS hosted.  Microsoft has blocked whole swathes of Amazon's Web Service platforms and Microsoft utterly refuse to unblock them.  I will raise this new development for Plusnet to the business.

@jab1 

You infer that Microsoft have / had a plot and the that plot is not pernicious.  Based on 18+ months of issues elsewhere, where the 'solution' being offered by our service provider is to move our hosting to Microsoft, I would suggest that the plot is devious, having the intent of pushing everyone to use Microsoft because that the only space not blocked by them

[#12]

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@Townman wrote:

---

---

@jab1 

You infer that Microsoft have / had a plot and the that plot is not pernicious.  Based on 18+ months of issues elsewhere, where the 'solution' being offered by our service provider is to move our hosting to Microsoft, I would suggest that the plot is devious, having the intent of pushing everyone to use Microsoft because that the only space not blocked by them

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Don't worry - I have had that thought for while, and I think you may have referenced earlier in this topic?

[#13]

@Townman re post #606

Well, I could do that but surely so could Plusnet! As I’ve said before (post #551), I would have hoped that by now a Plusnet team would have worked out exactly what is going on and be deep in discussion with Microsoft about fixing it. It also occurs to me that if Plusnet told Microsoft that sending emails as plain text got around their spam filter then the more likely outcome would be for Microsoft to plug that ‘loophole’.

As it happens, I sent an HTML email this morning To (not Bcc) 18 addresses (4 hotmail and 14 non-MS) and the 4 hotmail ones were rejected (the usual "550 5.7.515 Access denied"). Sending the same email as plain text to the 4 failures resulted in no rejections.

[#14]

@abitpedantic 

Unfortunately I have no idea what the business has been able to reproduce and send to Microsoft.  Similarly I do not know what 'odd-ball' possibilities they are considering.  From the testing I have done, I have yet to be able to recreate the issue.  Your reproduceable comparative scenario is solid gold, which this morning I have escalated into the business.

Given that this is a tad odd-ball, it is reasonable to ask if this has been considered in their investigation.  Out of many years of diagnosing complex issues, there reaches a point where everything reasonable which might give rise an issue has been eliminated, thereafter creative odd-ball considerations need to be explored.

[#15]

"M$ really, really have lost the plot"

How can something be lost if it never existed?