What Microsoft’s Year-Long Email Security Data Reveals About Protecting Your Inbox

What Microsoft’s Year-Long Email Security Data Reveals About Protecting Your Inbox

Email remains the primary vector for cyberattacks, and for good reason: it’s ubiquitous, it’s trusted, and one wrong click can compromise an entire organization. Microsoft recently published findings from a year-long benchmarking study of its Defender for Office 365 email security platform, covering data from June 2025 through June 2026. The report offers a rare look at the volume and types of threats that actually reach users, as well as how well the built-in protections perform. Here’s what stood out and what it means for anyone who uses email.

What Microsoft’s Data Shows

The study analysed millions of emails processed by Microsoft Defender across a broad set of enterprise customers. While the full report includes many metrics, three findings are especially relevant for everyday users and IT teams alike.

Phishing remains the dominant threat. Phishing emails accounted for the majority of malicious messages that Defender intercepted. Many of these were credential phishing attempts designed to look like legitimate notifications from services such as Microsoft 365, DocuSign, or package delivery companies. A significant portion also used social engineering tactics like impersonating executives or IT support. Microsoft notes that while basic phishing is often caught by spam filters, more sophisticated spear‑phishing and business email compromise attempts continue to evolve.

Malware attachments are less common but more dangerous. The share of emails containing malicious attachments was relatively small compared to phishing, but those that got through often carried ransomware or info‑stealers. Microsoft reports that its detonation chambers and attachment scanning blocked the vast majority of these before they reached users—but a small percentage bypassed initial filters, usually because the malware was served from a compromised legitimate site or used uncommon file types.

Spam volume is still enormous. Unsurprisingly, spam made up the bulk of all blocked email. Defender’s machine‑learning models improved spam detection rates over the year, but the volume of automated, low‑quality messages continues to rise.

Why This Matters

The numbers underline a simple truth: email security is a numbers game, and attackers are getting better at flying under the radar. The fact that even a well‑protected platform like Defender sees occasional successful bypasses means that no single tool is a silver bullet. For IT teams, the data points to the importance of layered defenses—and for individual users, it’s a reminder that the last line of defense is often human judgment.

It’s worth noting that Microsoft’s benchmarking focuses on Defender’s own telemetry, and the methodology has some inherent limits. For instance, the data likely undercounts threats that were caught before they ever reached Defender’s visibility (e.g., by other security tools in the customer’s environment). Still, the trends are consistent with what other vendors and industry reports have shown.

What You Can Do to Strengthen Your Email Security

The benchmarking results suggest several practical steps that both IT administrators and individual users can take right now.

Enable multi‑factor authentication everywhere. According to Microsoft, a large percentage of credential‑phishing attempts succeed only because MFA wasn’t in place. Had MFA been active, the stolen credentials would have been useless. If you haven’t already, turn on MFA for your email account and any other services that offer it.

Review your spam and phishing filter settings. Many email clients allow you to tweak sensitivity levels or add custom rules. At minimum, make sure that external email is clearly flagged (e.g., with a banner) so you can spot messages that impersonate internal senders.

Report suspicious emails quickly. How you report makes a difference. Microsoft Defender uses user feedback to train its detection models. If you receive a suspicious email, use the “Report Phishing” button in Outlook rather than just deleting it. Over time, this improves protection for everyone in your organization.

Be wary of unexpected attachments or links. Even if an email looks legitimate, double‑check the sender address and hover over links before clicking. When in doubt, visit the service directly by typing the address in your browser rather than clicking the link.

Consider enforcing stricter policies for shared mailboxes and distribution lists. These are often overlooked and can be a soft target for attackers. Restrict who can send to large groups and require approval for external mail.

Sources

• Microsoft. “Microsoft Defender email security benchmarking: Key insights from one year of data.” June 2026.
• Microsoft. “Clarity in complexity: New insights for transparent email security.” December 2025.
• Proofpoint. “Seeing the Full Picture: How to Measure Email Security Effectiveness the Right Way.” June 2026.

(Note: The benchmarking data referenced is from Microsoft’s own published reports. Individual results may vary depending on configuration and environment.)