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

[#1]

If you are sending from @*.plus.com it is surprising that you are able to send to anywhere at all.  The SPF configuration only permits sending via relay.plus.net from such addresses.

Whilst you chose to attempt something different you place yourself outside of the support envelope

[#2]

This was part of a business email to 9 recipients,  7 have been delivered,
the 2 hotmail.co.uk accounts have rejected as follows : 

One with the usual :

Spf= Pass , Dkim= Fail , DMARC= Pass


The other with a new response :

Reporting-MTA: dns; avasout-ptp-002 [84.93.230.235]
Received-From-MTA: dns; Clarelaptop [86.30.96.254]
Arrival-Date: Fri, 15 Aug 2025 12:42:49 +0100


Final-recipient: rfc822; xxxxxxxxx@hotmail.co.uk
Diagnostic-Code: smtp; 500 5.3.3 Unrecognized command 'Length: 6' [AM4PEPF00027A62.eurprd04.prod.outlook.com 2025-08-15T11:42:29.148Z 08DDD72A86D7E2FC]

Last-attempt-Date: Fri, 15 Aug 2025 12:42:49 +0100

[#3]

Hi Phillip,

I think this is potentially very useful, I will flag up.

A google on that error code here - The Cryptic NDR Codes Revealed! | Microsoft Learn - reports this...

5.3.3

Remote server has insufficient disk space to hold email. Check SMTP log.

Could that possibly indicate that the recipient's mail box / service is at capacity?

Though elsewhere I found...

5.3.3 System not capable of selected features

Selected features specified for the message are not supported by the destination system

However if either of these applied, one would expect ALL attempts to email the addressee to fail.

[#4]

Diagnostic-Code: smtp; 500 5.3.3 Unrecognized command

Given there is a lack of disk space message, that looks unlikely this time.

I agree this is very odd especially as the non MS recipient all worked.

[#5]

@Tim-J (post #514)  I've had an email to a group of 8 or 9 contacts where Outlook.com bounced but Hotmail didn't.

Then another day, for an email to the same group of contacts, Hotmail bounced but Outlook.com didn't.

@Townman (post #515) Do you have the NDR reports still? What do they indicate - suspected SPAM or DKIM failure?

Sorry, I can't find those particular NDR reports.  All I've got is some older ones to the same group of recipients in June.  Back then, SPF failed but DKIM passed, for both Outlook and Hotmail recipients together.  However, I think that PN and/or MS have tweaked things since then, so it's not very helpful?

@Townman If the latter, is there any detail, such as 'lookup timeout'?

No, just the same "doesn't meet the required authentication level" message quoted here many times.

@Townman Which Plusnet AVSOUT did they pass through?

avasout-peh-003 [212.159.14.19] for these particular ones.  Though failures to other Outlook/Hotmail addresses around the same time had other AVSOUTs, and as I say they may not be relevant to the more recent iteration of the problem.

Sorry not to be more helpful.

[#6]

Judging by the lack of posts over the last few days, I can only conclude that everyone on this forum has either lost interest or has given up on Plusnet.

I remember the Plusnet adverts a few years ago that had the slogan “Plusnet – we’ll do you proud”. Clearly, Plusnet are no longer the slightest bit interested in customer relations and would seem to be happy for everyone to have to move away to another email provider. Even the Superusers are keeping their heads down or is the prognosis so bad that they have been sworn to silence – very disappointing all round.

[#7]

No one has been sworn to silence.  There’s no update available at the moment.

[#8]

Gosh that is ominous.

[#9]

The complete lack of an official update for several weeks is exceptionally poor in terms of professional IT service management. Such an approach makes me feel that us service users are being strung along until the Greenby e-mail solution is in place.

If Plusnet is working on a resolution then they should at least share their investigation approach(es) with us. Giving service users reassurance during service failures is the professional approach. Silence, as we are experiencing, is unacceptable for this key aspect of service.

Imagine if this issue was taking place in a large corporate organisation with the same lack of communication - heads would roll.

[#10]

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@ebforum wrote:

Imagine if this issue was taking place in a large corporate organisation with the same lack of communication - heads would roll.

---

Doubtful,  "We're working on it." is the standard response..........

[#11]

I beg to differ from professional experience, senior management and the user community expected and deserved meaningful communications. Rightly so!

[#12]

In my professional experience, senior management and the user community wouldn't have a clue what you're talking about.

[#13]

I would not be communicating technical information. It would be along the lines of the key aspects of investigation in non-technical terms, who is undertaking and responsible for aspects of the investigation, advising on anticipated timeframes, giving assurances on when the next update communication will be sent out and so on.

[#14]

@Champnet
In my profession experience, senior management in a huge diverse multinational might not understand the nitty-gritty detail of a particular product division, leaving it to the management of that division.
But Plusnet is a tech company, not a diverse multinational. I've complete confidence their senior managers understand the problem. They are just keeping their heads down because they haven't been able to get meaningful answers from Microsoft, and they know it will soon be Greenby's problem, not theirs.

[#15]

@ebforum  In other words "We're working on it."