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Microsoft Outlook introduces new rules for high volume senders - What's changing for deliverability?

· 3 min read

As of May 5, 2025, Microsoft has implemented new, stricter authentication rules for senders who send more than 5,000 emails per day to Outlook.com, Hotmail.com, and Live.com addresses.
This move follows the steps taken by Gmail and Yahoo in 2024 and marks another step toward a safer, more trustworthy email ecosystem.

What does the new policy require?​

Microsoft now requires all high-volume senders to correctly implement the following authentication protocols:

  • SPF (Sender Policy Framework): The domain's DNS record must explicitly authorize the IPs allowed to send emails on its behalf.
  • DKIM (DomainKeys Identified Mail): Emails must be digitally signed to ensure message integrity.
  • DMARC (Domain-based Message Authentication, Reporting and Conformance): A DMARC record must be published with at least a p=none policy and must align with either SPF or DKIM (ideally both).

Messages that don’t meet these requirements will initially be delivered to the Junk folder.
Later on, Microsoft may outright reject them with SMTP error 550 5.7.515, indicating that the sending domain does not meet the required authentication level.

No official spam rate threshold… for now​

Unlike Google and Yahoo, which included a maximum spam complaint rate (typically under 0.3%) in their requirements, Microsoft has not yet provided any official guidance on this.
However, that doesn’t mean spam complaints don’t matter: high user-reported spam rates can still severely harm your sender reputation and deliverability - even without a specific threshold in place.

Additional recommendations for better deliverability​

In addition to the technical requirements, Microsoft recommends the following best practices:

  • Valid sender addresses: Make sure “From” and “Reply-To” addresses are real, receive replies, and reflect your actual domain.
  • Functional unsubscribe links: Always include a clear, visible unsubscribe link in marketing or bulk emails.
  • List hygiene: Regularly remove invalid or inactive email addresses to reduce bounce rates and spam complaints.
  • Transparency: Use honest subject lines and authentic content, avoiding misleading headers or sender names.

How to comply: Contactlab solutions for domain security​

If you're looking to quickly comply with the new authentication requirements from Microsoft (and from other providers like Google and Yahoo), you can contact your Contactlab sales/account representative.
Two solutions are available to help protect your domain and ensure reliable deliverability:

  • Brand Shield Base: Designed to ensure compliance with SPF, DKIM, and DMARC requirements, with guided setup and initial monitoring support.
  • Brand Shield Advanced: Includes all features of the Base version, plus a dedicated IP address, ideal for senders who want to isolate their reputation and gain greater control over deliverability.

Conclusion​

If you're sending more than 5,000 emails per day, it's crucial to check and update your domain’s DNS settings to comply with Microsoft’s new requirements.
This is an opportunity to strengthen your email security, protect your brand, and ensure optimal deliverability.

👉 Learn more from Microsoft’s official announcement:
Strengthening Email Ecosystem: Outlook’s New Requirements for High‐Volume Senders