Microsoft’s May 2025 Spam Filter Changes: Why False Positives Are Rising—And How to Fix Them

In May 2025, Microsoft rolled out significant changes to its spam filtering algorithms, impacting organizations worldwide. Many companies have since reported a spike in false positives—legitimate emails being flagged as spam or sent to quarantine. This post explains what changed, why it matters, and what IT admins and users can do to minimize disruptions.

What Changed in Microsoft’s Spam Filtering?

On May 5, 2025, Microsoft began enforcing stricter authentication requirements for all bulk email senders targeting Outlook, Hotmail, Live.com, and other Microsoft domains. The new rules primarily affect organizations sending more than 5,000 emails per day, but their ripple effects are being felt across many environments. 

Key changes include: 

·     Mandatory Authentication: All bulk senders must authenticate emails using SPF, DKIM, and DMARC protocols.

·     Stricter Enforcement: Emails failing these checks are more likely to be routed to spam or outright blocked.

·     Focus on Domain Hygiene: Microsoft now scrutinizes sender reputation, domain alignment, and message headers more aggressively. 

These changes are designed to combat phishing and spoofing, but they also make Microsoft’s spam filters more sensitive—sometimes at the expense of legitimate business communications.

Why Are False Positives Increasing?

Several factors contribute to the rise in false positives since the update:

·     Incomplete or Incorrect Email Authentication: If SPF, DKIM, or DMARC records are missing or misconfigured, legitimate emails may be flagged as suspicious.

·     Aggressive Filtering Policies:** Enhanced spam filter settings, such as Advanced Spam Filter (ASF) options, can inadvertently catch good emails.

·     Bulk Emailing Practices: Even transactional or notification emails from trusted sources can be marked as spam if they don’t fully comply with the new requirements.

·     Rapid Policy Enforcement: The transition period has left many organizations scrambling to update their DNS records and email-sending practices, increasing the likelihood of misclassification. 

How to Reduce False Positives in Microsoft 365

While no spam filter is perfect, there are several steps organizations can take to reduce the risk of legitimate emails being quarantined or marked as spam:

Audit and Update Email Authentication 

·     Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned with your sending domains.

·     Regularly review DNS records to confirm they reflect your current email infrastructure.

Fine-Tune Spam Filter Settings 

·     Review and adjust anti-spam policies in Microsoft Defender for Office 365.

·     Consider disabling overly aggressive ASF settings if they are causing excessive false positives.

·     Use the Tenant Allow/Block List to temporarily allow trusted senders or domains that are being misclassified. 

Leverage User and Admin Reporting

·     Encourage users to report false positives using the “Not Junk” button in Outlook or the Microsoft Message Add-in.

·     Submit misclassified emails to Microsoft through the Security portal to help improve filter accuracy over time.

Maintain Clean Mailing Lists

·     Remove inactive, duplicate, or invalid email addresses from your distribution lists to reduce bounce rates and improve sender reputation. 

Whitelist Trusted Senders (With Caution) 

·     Add key partners or internal domains to your organization’s allow lists, but use this feature sparingly to avoid security risks.

Monitor Quarantine and Junk Folders 

·     Regularly check quarantine and junk folders for legitimate emails, especially during periods of policy change.

·     Train users to recognize and escalate false positives promptly.

Proactive Steps for IT Teams 

Action Item                               Why It Matters

Audit SPF, DKIM, DMARC records

Prevents authentication failures and misclassification

Adjust spam filter policies

Reduces aggressive filtering that leads to false positives

Educate users on reporting process filter errors

Speeds up correction of filter errors

Monitor deliver ability metrics

Early detection of issues with legitimate mail flow 

Final Thoughts

Microsoft’s May 2025 spam filter overhaul is a double-edged sword: it improves security but can disrupt business communication if not managed carefully. By proactively updating authentication records, fine-tuning filter settings, and educating users, organizations can significantly reduce the risk of false positives and keep critical emails flowing smoothly.

For tailored guidance on optimizing your Microsoft 365 spam filtering, contact Moore Technology Consulting today.