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

[#1]

I got similar, over 16 hours before getting a delivery failure.  This was from K-9 Mail app on my tablet; still no delivery or failure notification from several sent from Thunderbird desktop.

As you can see below it is due to being on the blocklist as I think others have observed.

Extract from delivery failure email -

Diagnostic-Code: smtp; 550 5.7.1 Unfortunately, messages from [212.159.14.20] 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=11BC011725376C3B] [MN1PEPF0000F0DE.namprd04.prod.outlook.com 2025-10-04T08:06:43.212Z 08DE008B95EB9260]

Last-attempt-Date: Sat, 04 Oct 2025 09:06:43 +0100

[#2]

Yes, same server avasout-peh-004 [212.159.14.20]. While I was getting almost instant delivery failures, I rated the service dreadful because at least it was consistent so you knew where you stood. For me, this latest development (a delay of 16+ hours) has shifted it to unusable.

[#3]

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@Townman wrote:

The empirical observations suggest that the issue is predicated on the following...

1. Sending from @account.plus.com - sending from one's own hosted domain name does not seem to be an issue

---

Unfortunately, that's not the case. I run my own mail server for my own domain for which I have DMARC, SPF and DKIM set up  (verified again today) but email to hotmail.com addresses bounces (eventually, sigh) with:

Diagnostic-Code: smtp; 550 5.7.1 Unfortunately, messages from [212.159.14.20] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

[#4]

On a google search for 'Plusnet/microsoft email issues' I found this from the Plusnet community. From this I would assume there is unlikely to be a resolution. My one and only Plusnet customer has now moved to EE and Gmail. Taking the plunge and individually writing to her 200+ contacts to give them her new address.

This is copied directly from a Plusnet community post. Looks like it is created by AI but, nevertheless it seems to say it all. Is the Greenly migration complete as the post suggests.

What has happened to Plusnet emails?

AI Overview

Plusnet stopped providing new email, domain, and webspace accounts as of April 15, 2025, though some existing customers had their services moved to a new supplier, Greenby, for a limited time. While you can no longer get Plusnet email as a new service, existing customers with an email address tied to their broadband account have received a free migrated service for a period. However, you should strongly consider migrating to a different, independent email provider, such as Gmail, to avoid losing access to your email in the future, as your Plusnet account may have a limited lifespan. 

What is happening now:

• No new accounts:

  Plusnet no longer offers email, domain, or webspace services to new customers. 
• Migration to Greenby:

  Many existing Plusnet email accounts were migrated to a new supplier called Greenby. 
• Free service for a period:

  Customers who had their email linked to a broadband account on the migration date were given the migrated service for free for a limited time. 

Why you should migrate your email address:

• Future uncertainty:

  While your service might be covered for a period, it's not a permanent solution, and you could lose access to your email in the future. 
• Risk of loss:

  If Plusnet eventually discontinues the email service, any emails or registrations tied to that address will be lost forever. 
• Lack of control:

  Relying on your ISP for email is generally considered a risk, as the service's future is tied to your internet provider. 

What you should do:

1. Check your Plusnet account details:

   Review any communications or log into your Plusnet account to understand your current email service status and migration details. 
2. Choose an alternative email provider:

   Sign up for a free email account with a major provider like Gmail or Outlook.com. 
3. Update your online accounts:

   Log in to all your important online accounts and update your registered email address to your new one. 
4. Notify your contacts:

   Inform friends, family, and other contacts about your new email address. 

[#5]

@solodchin Do not believe everything AI tells you - it can be embarrassingly wrong.

The transfer to Greenby is still scheduled to happen, but not quite to the original timetable due to issues encountered when they attempted to move the first (small) tranche.

Can you please link to the Community post this is 'directly copied from' - it isn't, so you will have a problem there to start with.

I would pull that apart, line by line, if I had the time, but unfortunately I don't today, so will leave it for someone else. If it hasn't been dissected by tomorrow morning, I will comment further.

[#6]

@jab1 I never trust AI. Not a fan. However, it gets its info from somewhere 😉 Don't have the original to hand but I'll try and find it again.

Sorry folks. I wasn't from PN Community but an AI adjunct to a PN Post. I saw he PN Logo at the top of the screen and assumed the whole thing was from PN. Many apologies.

[#7]

Massively off topic, but I was reading somewhere about AI reporting something as true, based on a post on a generally good web site - except the article it took for granted and shared far and wide was published originally on the 1st April. 

[#8]

AI is not the 'great idea' Gates and co would like us to believe.

[#9]

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@solodchin wrote:

AI Overview

Plusnet stopped providing new email, domain, and webspace accounts as of April 15, 2025, ... 

---

In the very first sentence AI got it wrong ... Plusnet stopped these bells and whistles add-ons a long time ago.

People need to realise that AI is little more than a GIGO system (garbage in, garbage out).  Think of it as a super search and precise engine.  We have all seen the erroneous garbage written by bloggers and pundits; all that AI GIGO does is to deliver it to you quickly in sound bites which sound authoritative.  In some ways its just like BBC News, sounds good but only part of the real story.

[#10]

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@zopatista wrote:

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@Townman wrote:

The empirical observations suggest that the issue is predicated on the following...

1. Sending from @account.plus.com - sending from one's own hosted domain name does not seem to be an issue

---

Unfortunately, that's not the case. I run my own mail server for my own domain for which I have DMARC, SPF and DKIM set up  (verified again today) but email to hotmail.com addresses bounces (eventually, sigh) with:

Diagnostic-Code: smtp; 550 5.7.1 Unfortunately, messages from [212.159.14.20] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

---

That is a different issue arising from the recent proliferation of spam.

The substantive subject of this thread is the inconsistent treatment of emails sent to just one Microsoft addressee or multiple addressees.  In that context I stand by the statement.

Your issue is a different issue, so indeed my statement does not apply.

[#11]

@Townman I did apologise to jab1 for suggesting that the comments came from PN. I also explained that I am no fan of AI and your description go GIGO is pretty much correct - the only change to that would be, where AI is concerned AIGO would be better - Anything In Garbage Out. I only wish I could rid myself go AI completely.

[#12]

@solodchin 

Cheers!!

My response was not intended to be aimed at you but at AI, seeking to clarify for all that AI should not be read as being 'gospel' on matters.  Sorry if it came across as being personal.

There is far too much reliance being placed on AI by folks because of the 'intelligence' connotation implying authoritative thought and reflection; it is not.  I like your AIGO and I am tempted to plagiarise it, before you can copyright it!!

[#13]

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@Townman wrote:

That is a different issue arising from the recent proliferation of spam.

Ah, sorry. It's a very long thread at this point and put the wrong contexts together.

So, at least my problem is one that should resolve itself automatically I assume.

[#14]

@zopatista 

Yes, black listing should clear in 24-48 hours.  You can check the black listing of your domain name using MXTOOLSBOX.

[#15]

The delivery failure message reported by @zopatista gives the IP address of one of Plusnet's avasout servers, so isn't that the one that needs to be got off the block list and there is no evidence of progress being made on that or indeed regarding any of the other Plusnet servers on block lists (as captured in #681 and a couple of subsequent posts).