How to Prevent Microsoft 365 Blocks: Proactive Tips for Cold Email Deliverability

Cold email remains one of the most effective channels for startups and sales teams to drive new business. But as inboxes become more crowded and email providers like Microsoft 365 get smarter at filtering out unwanted messages, deliverability has become a make-or-break factor for outreach success.
If you’ve ever experienced a sudden drop in response rates, or worse, a surge in soft bounces and blocked emails, Microsoft 365’s filtering system could be the culprit. With over 345 million active users, Microsoft 365 is one of the world’s most widely used business email platforms. Its robust security and anti-spam features are great for users, but they can be a major headache for legitimate senders who don’t follow best practices.
This guide will show you how Microsoft 365 blocks work, why they happen, and most importantly, what you can do to proactively prevent them. By following these proven deliverability strategies, you’ll keep your cold emails landing in the inbox and your pipeline full.

Understanding Microsoft 365 Blocks

What is a Microsoft 365 Block?

Microsoft 365 blocks occur when the platform’s security filters identify your messages as potentially harmful or unwanted. These blocks can manifest as soft bounces (temporary delivery failures) or hard bounces (permanent failures), but most cold emailers encounter soft bounces first. A soft bounce typically means your email wasn’t delivered, but you might be able to fix the issue and try again.

Common Reasons for Blocks

• High sending volume or frequency spikes
• Technical misconfigurations (SPF, DKIM, DMARC)
• Poor sender reputation or engagement signals
• Spammy content, links, or attachments
• Recipient complaints or manual reporting

How Microsoft 365 Filters Work

Microsoft 365 uses a combination of reputation scoring, content analysis, and recipient feedback to decide whether to deliver, quarantine, or block emails. Their algorithms are updated regularly, and even small missteps can trigger a block, especially for cold outreach campaigns.

Why Blocks Happen: Key Triggers

Sending Behavior

If you suddenly ramp up your sending volume or frequency, Microsoft 365 may view this as suspicious. Sending hundreds of emails from a new inbox or domain, or sending to many recipients who haven’t engaged with you before, can quickly trigger soft bounces.

Technical Misconfigurations

Missing or misconfigured DNS records (SPF, DKIM, DMARC) are a major cause of deliverability problems. Microsoft 365 checks these records to verify your identity. If they’re missing or incorrect, your emails are more likely to be filtered or blocked.

Reputation and Engagement Signals

Microsoft 365 tracks how recipients interact with your emails. Low open rates, high bounce rates, or frequent spam complaints can damage your sender reputation. Over time, even a few negative signals can add up and lead to blocks.

Content Pitfalls

Spammy language, excessive links, large attachments, or misleading subject lines can all trigger Microsoft 365’s filters. Even if your technical setup is perfect, poor content can still get you blocked.

Proactive Strategies to Prevent Blocks

1. Set Up Domains and Inboxes Correctly

• Use reputable domain registrars and email providers.
• Configure SPF, DKIM, and DMARC records for every sending domain.
• Avoid using brand-new domains for high-volume outreach; age your domains first.
• Limit the number of inboxes per domain (Mailpool recommends 3/inbox/domain).

2. Warm Up Your Inboxes

• Start with low sending volumes (10–20 emails/day/inbox).
• Gradually increase your daily sending over 3–4 weeks.
• Use warm-up tools or services to simulate real conversations and engagement.

3. Maintain Clean Lists and Monitor Engagement

• Only email verified, opt-in contacts.
• Regularly remove unengaged or bounced addresses from your lists.
• Monitor open rates, reply rates, and bounce rates to spot issues early.

4. Avoid Common Content Triggers

• Write clear, relevant subject lines, and avoid clickbait.
• Limit links and attachments in your emails.
• Personalize your outreach to boost engagement.
• Avoid spammy words and formatting (e.g., ALL CAPS, excessive punctuation).

5. Manage Sending Volume and Frequency

• Spread out your sends over time, don’t blast all at once.
• Rotate sending between multiple inboxes/domains if possible.
• Monitor for any sudden spikes in volume or frequency.

Technical Setup Checklist

• SPF Record: Authorizes specific servers to send on your behalf.
• DKIM Record: Digitally signs your emails for authenticity.
• DMARC Policy: Instructs providers how to handle unauthenticated mail.
• DNS Configuration: Ensure all records are correct and propagate properly.
• Dedicated vs. Shared IPs: Dedicated IPs offer more control but require careful warming; shared IPs are lower risk for beginners.
• Microsoft-Specific Recommendations: Follow Microsoft’s published sender guidelines and continuously check for updates.

Monitoring & Troubleshooting

Detecting Deliverability Issues

• Monitor bounce and complaint rates daily.
• Use deliverability tools to test inbox placement and spam scores.
• Set up alerts for soft bounces or sudden drops in open rates.

What to Do If You Get Blocked

• Pause your campaigns immediately.
• Review your DNS and authentication records.
• Clean your lists and remove problematic addresses.
• Reduce sending volume and frequency.
• Reach out to Microsoft support if necessary.

Conclusion

Microsoft 365 blocks don’t have to derail your cold email strategy. By understanding why blocks happen and taking proactive steps to prevent them, you can keep your outreach on track and maximize your results. Remember to:

• Set up domains and authentication records correctly
• Warm up new inboxes slowly
• Clean your lists and monitor engagement
• Avoid spammy content and sending patterns
• Act quickly if you spot deliverability issues

Ready to take your cold email deliverability to the next level? Book a Demo with Mailpool and see how our platform can help you maximize inbox placement, automate deliverability management, and scale your outreach with confidence.