TalkTalk Email Sending Errors to Microsoft Emails

[Fingers]

Some emails sent via the TalkTalk domain are not being delivered but are bounced back due to a failure in TalkTalk's authentication process.
This is because TalkTalk's DKIM signing fails due to not corresponding with their 5322.domain.

Error message from Outlook.com, error 550.5.7.515, access denied.
(also similar error messages from other Microsoft controlled email systems e.g. Hotmail.co.uk).

Error message received:
outlook-com.olc.protection.outlook.com[52.101.9.25] said: 550 5.7.515
Access denied, sending domain TALKTALK.NET 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. To learn how
to fix this see: https://go.microsoft.com/fwlink/p/?linkid=2319303
Spf=Pass , Dkim= Fail , DMARC= Pass [DB9P195MB2063.EURP195.PROD.OUTLOOK.COM
2025-09-01T15:16:03.651Z 08DDE92A155C2C50]
[LV3P220CA0014.NAMP220.PROD.OUTLOOK.COM 2025-09-01T15:16:03.692Z
08DDE93589CAE798] [BL6PEPF00022572.namprd02.prod.outlook.com
2025-09-01T15:16:03.694Z 08DDE77FE095163E] (in reply to end of DATA command)

Is this a problem that only TalkTalk can correct?

[mandisa1-TT]

Hi there @Fingers i agree with @nameunknown_ please try using webmail and should you still be experience an issue please post back and we will be able to further look into this for you.

 

[nameunknown_]

@Fingers 

TalkTalk (and Microsoft) ought to be investigating the cause of the DKIM failure. Unless TalkTalk has evidence to show that it is caused by an email application, TalkTalk should not be 'hiding' behind saying TalkTalk does not support use of third party applications, because that is just a cop-out.

The same email composed in an email application and in webmail would almost certainly have different DKIM signatures, so sending the same email from both at the same time, as you have suggested, may not be a useful comparison.

The best way to determine whether the DKIM failure ever occurs when using webmail would be to consistently use webmail for sending emails to Microsoft domains. If you can show that it does, then TalkTalk should investigate, because TalkTalk would no longer be able to use the reason for not investigating that it currently is.

[Fingers]

Unfortunately the problem isn't resolved, I've just had another bounce back from an outlook.com email address with the same error as before: DKIM Fail. But the issue is intermittent, as I have since re-sent from both Outlook 2019 and TalkTalk webmail but neither bounced back this time. So it is not certain whether or not the issue also exists when using webmail as that would have to be checked by sending at exactly the same time that it fails via Outlook.

[sabelo-TT]

@Fingers , not a problem and @nameunknown_ thank you for helping out 🙂 

[Fingers]

@nameunknown_ 

Thanks for that information about the sort order, I'm used to other bulletin boards, etc. where all is in chronological order from the original post. I was confused by having the original post first followed by the latest post next and so on back through the order.

[nameunknown_]

@Fingers 

By default, after the opening post, all other posts in a topic appear in the order newest to oldest from the top down.

You can change this on a topic by topic basis using the sort option, which appears under the Replies count that is located on the left under the opening post.

Alternatively, you can change the the default order for all topics in your settings.

Go to:

https://community.talktalk.co.uk/t5/user/myprofilepage/tab/personal-profile

select Preferences -> Linear Layout

change the setting for:

Linear Format: Sorting order within topics

[mandisa1-TT]

Hey there @Fingers Thank you for updating us the issue is now resolved. How are your replies appearing? as on my end the most recent replies appear at the top

[Fingers]

I haven't altered anything, but my emails to hotmail and outlook mail have suddenly started being delivered normally again!
So outlook.com, hotmail.com and hotmail.co.uk email deliveries appear to be OK again now; I hope it stays that way but don't know what has resolved the issue, perhaps TalkTalk have introduced some changes, does anyone know?

On a related issue, why do all the replies here appear in such a mixed up reverse order, it makes it very difficult to follow the thread; does the same illogical display order happen for all of you too?

[nameunknown_]

@Fingers 

That shows the domain and selector for DKIM are @talktalk.net and mail1, for which there is a valid DKIM record containing a public key to validate the DKIM signature.

If the email you sent to the @outlook.com email address also has that domain and selector combination for DKIM (there is no reason to think otherwise) , neither of the likely reasons for rejection that I previously suggested appear to be the reason for rejection, because there is no mismatch between the sender domain and the domain for DKIM, and there is a valid DKIM record.

Further investigation to identify the cause of the DKIM failure requires sight of what was actually sent from the TalkTalk server to the Outlook server.

Have you tested using a simple plain text email with no attachments?

With how many Microsoft accounts have you seen rejections?

[Fingers]

Hi nameunknown_
When I send from my talktalk.net email to other emails (gmail.com & lineone.net), the internet header includes:
dkim=pass header.i=@talktalk.net header.s=mail1 header.b=ou4ap0vE  [gmail]
or
dkim=pass header.d=talktalk.net header.s=mail1 header.i=@talktalk.net  [lineone]
But these emails all go into my spam folders and even if I set Outlook 2019 to not treat them as spam and send again, they still go into spam which is another puzzle?
Thus dkim=pass when sending to addresses which are not outlook.com nor hotmail.co.uk when sent via Outlook 2019.
N.B. I'm using servers smtp.talktalk.net to send and mail.talktalk.net to receive

[nambuso-TT]

Thanks for supporting @nameunknown_ . 

[nameunknown_]

@Fingers 

That looks like the header of an email that has been composed, but not sent and received. The DKIM header (along with other additions) would be appended by the server.

In order to view the header after the email has been received, you need to send it to another email account of yours, to which you can successfully send it without it being rejected.

The fact that you sent the email using an @talktalk.net email address makes the issue something of a mystery. The header that I put in my previous comment was from an email sent from an @talktalk.net email address (using Microsoft Outlook 2007) to an @outlook.com email address. It was not rejected, hence I received it, and looked at the header.

Look at the DKIM header of an email sent from your @talktalk.net email account, and successfully received in another email account of yours. The attributes in the DKIM header that are of particular interest are the domain (d) and the selector (s). As you can see the header of my email has:

d=talktalk.net; s=mail1;

There is a valid DKIM record for the domain an selector combination.

Are using the TalkTalk smtp.talktalk.net server to send the email?

[nambuso-TT]

Hi there @Fingers I am very sorry to hear this. So you can say that you are unable to send emails?

[Fingers]

Hi nameunknown_
I was sending from my talktalk.net email, i.e. <myname>@talktalk.net, thus your suggestion does not seem to be the reason for failure. I haven't tried using a lineone email to send to hotmail or outlookmail as I expected that to fail similarly to my talktalk email.
As per your suggestion I viewed the email header in Outlook 2019, by opening the email, clicking on File, then on Properties. But this was all the information provided (asterisks replacing sensitive information) which doesn't seem to reveal much:

Internet headers
From: "************ <************@talktalk.net>
To: "************" <************@outlook.com>
Subject: ************
Date: Tue, 15 Jul 2025 09:13:38 +0100
Message-ID: <001f01dbf560$601cb040$205610c0$@talktalk.net>
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----=_NextPart_000_0020_01DBF568.C1E11840"
X-Mailer: Microsoft Outlook 16.0
Thread-Index: AQI4BYgeAnw2kwHYg1jnMZzjzB2Vzg==
Content-Language: en-gb
X-MS-TNEF-Correlator: 00000000AB2C3746BE0CFF41B3F0CCA4EEF83C470700D9539C2261A6BB45B9DAB62C7081B3C101002400FFFF000081DC8C2D469A1446A4A772279E50A8D2000000003C560000

So what to do next?

[nameunknown_]

@Fingers 

I am guessing that you are using a lineone.com email account (having seen a reference to that in another thread of yours). If that's the case, then the likely issue here is either that that the value for the domain (d) attribute in the DKIM header of the email does not match the domain in the 5322 From: header, e.g. the former is talktalk.net and the latter is lineone.com, or if they do match, i.e. they are both lineone.com, the DKIM record is missing or incorrect.

If you have another email account to which you can successfully send an email without it being rejected for a DKIM failure, looking at the DKIM header would be useful.

To view the header in Outlook 2019, open the email, click on File, then click on Properties.

Look for something like:

DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=talktalk.net; s=mail1;
t=1756764536; bh=PyrA3OTi5k0W4ZfgX6WSyaKQBR5gFRLOnoK0yQGurPI=;
h=From:To:Subject:Date:From;
b=q/xbJ7cxtiXUSBfNJcEZG5jk6laLEKPU9SD7fhaHNcdMXPDKFArBdL7yy/H1I7Q/X
qVBl+9gSyNkBznRoeHURolRWIj9SsSg/JFOIuLFRLqUbbFdBSVYUbX73OLbD/mef7P
1GdXcEEc6X6yzIPROBZNkeZsnx5PP6xvEYxH7sPZq74S8r/jGug/9cRlvjNZAXidZ4
PdvrQdWCjw7V4thHIYRpZBXhPsvGoq0PFmX4vEwCJsJO+y5t2Xxd2WPo+hXqwunMwH
Kd/7+xBvkOZcds6p309LJ0xhHm7ImIDL+mlqGsQ5C8SqgzNNxih2eiSg8Mc7puramn
qlowwF4EUBkcg==

If it contains d=talktalk.net, and the domain in the 5322 From: header is lineone.com, then the rejection is due to the mismatch. If it contains d=lineone.com, then the rejection is due to an issue with the DKIM record.

If the domain (d) is set to lineone.com, then along with the selector (s) from the DKIM header, you can check whether the DKIM record exists using a site such as https://mxtoolbox.com/dkim.aspx .

Whatever the reason for the rejection, this is something that the team managing the servers need to resolve. The staff on the community ought to escalate the issue to technical support for investigation.

I think you know as well as I do that whilst this issue occurs using an email application but not webmail, the issue is not being caused by the email application. However,the community staff may advise you use webmail, and say support is not provided for issues with using third party applications. If that happens, you are left with the option of calling technical support.

[Fingers]

Hello Mandisa,

I've now sent via TalkTalk webmail to both Outlook and Hotmail email addresses and haven't had a bounce back so that is a good sign; but neither have I received any replies yet.
So the problem occurs when sending from Outlook (part of Office 2019) on a Windows 10 PC, but it is only Microsoft owned email systems that have the problem where their requirements appear more stringent, all other emails I send are received OK.
There must be a difference somehow, maybe an alternative TalkTalk domain used for email clients as opposed to that used for webmail which causes the Dkim failure?

[Mandisa-TT]

Hi @Fingers can you please confirm if you are able to send the email using Talktalk webmail.