How to Configure Microsoft 365 Anti-Spam Filtering Properly (Step-by-Step)

Why Default Settings Are Not Enough

Out of the box, Microsoft 365 catches most obvious spam but leaves gaps. Properly tuned policies significantly reduce phishing, spoofing, and business email compromise attempts.

Layer 1: Email Authentication (Do This First)

Before tuning filters, confirm these DNS records are in place:

Without DMARC at least p=quarantine, spoofed emails from your own domain can still reach inboxes.

Layer 2: Anti-Spam Policy Tuning

In Microsoft Defender > Email and Collaboration > Policies > Anti-Spam:

• Bulk email threshold: Lower from default 7 to 5 for stricter filtering
• Spam action: Move to Junk folder (or quarantine for tighter control)
• High-confidence spam: Quarantine
• Phishing: Quarantine
• High-confidence phishing: Quarantine and notify admin

Layer 3: Anti-Phishing Policy

• Enable impersonation protection for key executives
• Add your domain and executive names to monitored senders
• Enable mailbox intelligence
• Set spoofed sender action to quarantine

Layer 4: Safe Links and Safe Attachments

• Enable Safe Links for all users — rewrites URLs and checks on click
• Enable Safe Attachments — detonates attachments in sandbox before delivery
• Both require Microsoft Defender for Office 365 Plan 1 or higher

Quarantine Review Routine

• Assign a staff member to review quarantine weekly
• Release false positives and add to safe sender list
• Report false negatives (spam that got through) to Microsoft

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