Outlook's New Email Authentication Requirements for High-Volume Senders Effective May 2025​

Microsoft has announced that starting May 5, 2025, Outlook will implement stricter email authentication requirements for high-volume senders. This initiative aims to enhance email security and reduce spam and phishing attempts.

Based on Microsoft’s latest update for Outlook and Microsoft 365, the DMARC policy setting (whether "none", "quarantine", or "reject") no longer affects how Microsoft handles unauthenticated emails,  especially for high-volume senders.

What This Means:

Starting May 2025, Microsoft will begin enforcing its own DMARC-like checks, regardless of the domain's published DMARC policy. So, even if your DMARC policy is set to none (traditionally used for monitoring), Microsoft will still reject unauthenticated emails.

New rule: If your email fails DMARC and you send more than 5,000 emails per day, Microsoft will bounce those emails with this error:

"550 5.7.15 Access denied, sending domain [yourdomain.com] does not meet the required authentication level."

Authentication Requirements

To ensure uninterrupted email delivery, make sure your domain meets the following:

• SPF (Sender Policy Framework):
  Your DNS records must include authorized IP addresses or hosts. The email must pass SPF validation.
  
  
• DKIM (DomainKeys Identified Mail):
  Emails must be cryptographically signed with a valid DKIM key to confirm integrity and authenticity.
  
  

• DMARC (Domain-based Message Authentication, Reporting, and Conformance):
  Your domain must have at least a p=none DMARC policy, aligned with either SPF or DKIM (ideally both) to pass.
  
  

These updates are part of broader industry efforts to promote email trustworthiness and transparency.


Recommendations for High-Volume Senders

1. Perform a Full Domain Audit
   Identify all services and platforms sending email on behalf of your domain (e.g., CRMs, newsletters, support platforms).
   
   
2. Authenticate All Sources
   Ensure each sending source is configured with SPF and DKIM, and that these align with your "From" domain. If DKIM is missing, prioritize setting it up.
   
   
3. Use a DMARC Monitoring Tool
   Use a platform like EasyDMARC to monitor authentication results, identify misaligned senders, and receive actionable reports.
   
   
4. Fix Misalignments Quickly
   If you detect sources failing DMARC due to misalignment, reconfigure SPF/DKIM settings for those services or isolate them under subdomains.
   
   
5. Gradually Move Toward a Stricter DMARC Policy
   Once you're confident all sources are properly authenticated and aligned, consider moving to quarantine or reject to fully protect your domain from spoofing.
   
   You can check out Microsoft’s official article here: