Security Policy Errors

How to Fix Microsoft 365 Remote Server DMARC Authentication Error 550-5.7.26

Anum Mushtaq Oct 23, 2026 868 views

Question details

Users are unable to send emails between addresses on the same domain due to a DMARC policy failure.

How to Fix Microsoft 365 Remote Server DMARC Authentication Error 550-5.7.26

Product: Microsoft 365
Device & OS: not provided
Scenario: Sending internal emails between user addresses hosted on the same domain within a Microsoft 365 environment.
Observed behavior: The sent email fails to deliver and returns a 550-5.7.26 bounce-back error, stating that the message is not authenticated under the domain's DMARC policy.

Before you start

Ensure you have global administrator access to the Microsoft 365 admin center and administrative credentials for your domain's DNS hosting provider.

Solution 1Recommended

Verify and Update SPF and DKIM DNS Records

Publishing the correct SPF and DKIM records in your DNS settings is required for outbound Microsoft 365 mail to pass DMARC authentication checks.

DMARC relies on SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) to verify the sender's identity. If either of these is missing or misconfigured for Microsoft 365, internal emails may be rejected by your own domain's security policies.

Enable DKIM in Microsoft 365

Log in to the Microsoft 365 Defender portal. Navigate to Email & Collaboration > Policies & Rules > Threat policies > DKIM. Select your domain and toggle the option to enable DKIM signing.

Publish DKIM CNAME records

If DKIM is not yet set up, Microsoft 365 will generate two CNAME records. Copy these records and add them to your domain registrar's DNS management console.

Check SPF authorization

In your DNS management console, review your TXT record for SPF. Ensure it explicitly includes 'v=spf1 include:spf.protection.outlook.com -all' to authorize Microsoft 365 to send emails on your domain's behalf.

Verify and Update SPF and DKIM DNS Records

DNS Propagation Time: Any changes made to your domain's SPF, DKIM, or DMARC DNS records can take anywhere from 1 to 48 hours to fully propagate across the internet.

Solution 2

Review DMARC Policy and Mail-Flow Rules

Evaluate your domain's DMARC strictness and check for custom Exchange transport rules that might be breaking email authentication.

Solution 3

Open a Microsoft 365 Support Request

If all DNS records are correctly published and the DMARC error persists, escalate the issue to Microsoft 365 technical support.

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Frequently Asked Questions

What does Microsoft 365 error 550-5.7.26 mean?

Error 550-5.7.26 means that an email was rejected by the receiving server because it failed DMARC authentication. This happens when the sender's identity cannot be verified through SPF and DKIM records, triggering the domain's security policy to block the message.

Why are emails between my own domain addresses failing DMARC?

Internal emails can fail DMARC checks if they are routed through external third-party services (like cloud signature managers or anti-spam filters) before being delivered back to your Microsoft 365 inbox. This extra hop can alter the email headers and break the DKIM signature.

How do I check if my DKIM and SPF records are working?

You can use free online tools like MxToolbox or Google Admin Toolbox to verify your domain's DNS records. Alternatively, use the Microsoft 365 Defender portal's threat policies section to confirm the status of your DKIM setup directly.

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