Emails from Outlook account to other Outlook addresses now bouncing back as undeliverable

Question

Emails from Outlook account to other Outlook addresses now bouncing back as undeliverable

David L 0

This has just started happening and afaict is only affecting emails where the recipient is another Outlook address.

I am getting the following error message:

|| | -------- | More Info for Email Admins

More Info for Email Admins

|| || Status code: 550 5.7.23 This error occurs when Sender Policy Framework (SPF) validation for the sender's domain fails. If you're the sender's email admin, make sure the SPF records for your domain at your domain registrar are set up correctly. Office 365 supports only one SPF record (a TXT record that defines SPF) for your domain. Include the following domain name: spf.protection.outlook.com. If you have a hybrid configuration (some mailboxes in the cloud, and some mailboxes on premises) or if you're an Exchange Online Protection standalone customer, add the outbound IP address of your on-premises servers to the TXT record. For more information and instructions about configuring SPF records see Customize an SPF record to validate outbound mail sent from your domain and also External Domain Name System records for Office 365.

|| | -------- | Original Message Details

Original Message Details

|| || || | -------- | Created Date:

|| | -------- | 08/11/2026 09:45:08

Created Date:

|| || 08/11/2026 09:45:08

|| || Sender Address:

|| || [redcated]

|| || Recipient Address:

|| || [redcated]

|| || Subject:

|| || RE: Ciao bello

|| || Error Details

|| || || | -------- | Error:

|| | -------- | 550 5.7.23 The message was rejected because of Sender Policy Framework violation -> 550 5.7.1 Message rejected: Message lacks SPF/DKIM/DMARC Auth Records (AS9031519) [DU2PEPF00028D07.eurprd03.prod.outlook.com 2026-11-08T09:45:12.048Z 08DF3BECCB6625DE]

Error:

|| || 550 5.7.23 The message was rejected because of Sender Policy Framework violation -> 550 5.7.1 Message rejected: Message lacks SPF/DKIM/DMARC Auth Records (AS9031519) [DU2PEPF00028D07.eurprd03.prod.outlook.com 2026-11-08T09:45:12.048Z 08DF3BECCB6625DE]

|| || Message rejected by:

|| || DU2PEPF00028D07.mail.protection.outlook.com

Any ideas?

Many thanks

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3 answers

Your answer

Answer 1

David L 0

Emily

Many thanks for the response.

Yes it is a custom domain, albeit I typically send closer to 5 emails per day than 5,000.Afraid I don't have a clue what SPF, DKIM, and DMARC are and slightly hesitant about getting too deep into the weeds and making changes to settings etc in case I create an even bigger problem, as I won't have the technical expertise to fix it afterwards.

In your experience are MSFT likely to resolve this relatively quickly or not?

Best

David

0 comments

Answer 2

David L 0

Emily

Many thanks for the response.

Yes it is a custom domain, albeit I typically send closer to 5 emails per day than 5,000.Afraid I don't have a clue what SPF, DKIM, and DMARC are and slightly hesitant about getting too deep into the weeds and making changes to settings etc in case I create an even bigger problem, as I won't have the technical expertise to fix it afterwards.

In your experience are MSFT likely to resolve this relatively quickly or not?

Best

David

EmilyS726 279.1K Reputation points Independent Advisor

2026-11-08T17:15:05.0466667+00:00

As independent advisors, we are not entitled to any insider intel, especially if it is an emerging issue.

However, I did find out from another user I was helping, who managed to hear back from Microsoft support. They acknowledged the issue and mentioned they were hoping to fix this within the next few days.

So, if making changes on SPF, DKIM and DMARC seems like a challenge, I suggest that you wait for the requirements for these to go back to the 5000/day threshold.

But for the long term, it is best to consider setting them up if possible (start with your domain host because that is where DNS records are kept; they should be able to help you), because these are best practices anyway for email hygiene. A lot of email service providers require these for incoming emails. A few years ago these were not required at all, then they started soft requirements for high-volume senders, then hard requirements... all because scammers are advancing their spoofing techniques too. So I can imagine the threshold will only get lower as time goes.
David L 0 Reputation points

2026-11-08T17:23:47.5233333+00:00

Thank you!
EmilyS726 279.1K Reputation points Independent Advisor

2026-11-08T17:30:00.1566667+00:00

You are welcome.

Also, if possible at all, work with your domain host to enable DMARC. That alone might resolve the issue before Microsoft fixes on their end. It is very easy to enable and takes a few hours to propagate.

Answer 3

Are you using custom domain?

Hello

I am seeing a sudden increase in similar reports from other users in this forum as well.

Unfortunately, I cannot find any official information from Microsoft acknowledging this issue. Microsoft's published requirements for SPF, DKIM, and DMARC apply to senders sending more than 5,000 emails per day. However, based on the sudden increase in reports yesterday, I suspect these requirements may now be affecting low-volume senders as well. This is just my observation, not something Microsoft has officially confirmed.

I helped several users with this issue yesterday in this forum. One interesting thing we noticed is that email headers may show DMARC as passing, even when the sending domain does not have a published DMARC record. We were able to identify this by checking the domain using a free tool such as https://mxtoolbox.com/.

In some cases, once the missing DMARC record was properly configured and verified, emails started coming through again.

For this reason, I strongly recommend setting up SPF, DKIM, and DMARC records for the affected sending domains.

If you need help checking these records, I am happy to do so. I would just need to know the sending domain name. You do not need to share the full email address.

For now, verifying and correcting these authentication records is the best troubleshooting step I can recommend until Microsoft provides more official information.

Note: These are non-Microsoft websites. They appear to provide useful information, but be cautious of advertisements or downloads for potentially unwanted software. You do not need to download anything to use the domain-checking tools.

Also, news article about this issue: https://www.neowin.net/news/microsoft-outlook-spf-validation-error-blocks-email-sending-and-receiving/