Does Microsoft Defender replace the need for email security gateways?

Microsoft Defender for Office 365 has become the default email protection for many organizations, leaving business leaders wondering whether they still need a dedicated email security gateway.

That question matters because gateways historically filtered email at the perimeter; now that Defender is bundled into Microsoft 365, many assume those extra layers are unnecessary.

In practice, Defender delivers strong native defenses while gateways provide complementary capabilities that close blind spots, simplify compliance, and strengthen resilience. Pairing Microsoft Defender with Mimecast delivers protection without compromise.

How does Microsoft Defender process inbound and outbound email?

Microsoft Defender processes email inside Microsoft 365, scanning inbound and outbound messages once they enter or leave the tenant. While this protects against many known threats, it differs from a gateway that filters mail at the perimeter before it reaches the Microsoft environment.

For example, an attacker sends a malicious invoice. With Defender, the message lands in the Microsoft environment first before being analyzed and quarantined. With a gateway, that message is intercepted before it even touches the tenant, lowering exposure risk.

This difference also affects outbound controls, where perimeter enforcement can apply DLP and encryption policies before messages ever leave your domain.

Mimecast operates at the perimeter, filtering mail before it reaches Microsoft 365. That earlier checkpoint reduces exposure risk and provides stronger outbound safeguards, complementing Defender’s in-tenant scanning.

What protections does an email security gateway provide beyond Microsoft Defender?

Email security gateways provide protections beyond Microsoft Defender by offering granular controls for spam filtering, policy enforcement, encryption, and continuity. Defender’s focus is primarily on malware and phishing, while gateways add capabilities that reduce risk and improve manageability.

Comparison at a glance:

• Spam control: Gateways filter both malicious and unwanted bulk mail, while Defender prioritizes known threats.
• Encryption: Gateways offer simple policy-based encryption; Defender requires more complex setup.
• Continuity: Gateways keep mail flowing during outages; Defender does not.
• Policies: Gateways allow detailed rules for senders, content, and attachments.

Gateways also provide user-friendly quarantine releases and delegated workflows that cut help-desk tickets and speed resolution.

And independent analysts agree: the Forrester Total Economic Impact™ (TEI) study noted that “using an enterprise email security solution such as Mimecast in tandem with native email security infrastructure can offer greater efficacy and efficiencies than native-only email security alone while reducing concentration risk and increasing reliability.”

Mimecast’s Secure Email Gateway combines these administrative strengths with advanced detection, complementing Defender by reducing inbox noise, simplifying encryption, and providing a consistent security layer across all mail traffic.

Does Microsoft Defender scan emails before or after delivery?

Microsoft Defender scans emails after they are delivered into the Microsoft 365 environment. This means harmful content enters the tenant before being quarantined, while gateways analyze mail before delivery and block it at the perimeter.

Why it matters:

• Defender: Stops many threats but only after they’ve touched the Microsoft environment.
• Gateways: Prevent delivery entirely, shrinking the attack window.

The layered approach is now de facto. In Forrester’s Security Survey, 63% of director-level security leaders said their firm currently uses two or more vendors in its content security environment—a category that includes email, messaging, and collaboration security. This reflects the growing recognition that relying on one in-tenant solution alone leaves gaps that can be closed with complementary tools.

Mimecast addresses this gap by filtering at the perimeter, ensuring malicious messages never reach inboxes in the first place.

How do email gateways help manage spam and graymail compared to Defender?

Email security gateways manage spam and graymail more effectively than Microsoft Defender by applying advanced filters and customizable rules. Defender focuses on malicious content, but bulk promotional messages often slip through and clutter inboxes.

• Employee perspective: A cluttered inbox makes it harder to spot legitimate business email, leading to delays and frustration.
• IT perspective: More user complaints, more wasted time cleaning up junk, and fewer cycles spent on real security tasks.

Gateways can safely enable user-level digests and controlled unsubscribe options that reduce noise without weakening security.

Mimecast complements Defender by reducing graymail, offering fine-grained spam controls, and giving administrators flexible options. This keeps inboxes clean and allows Defender to focus on high-risk threats.

Can Microsoft Defender guarantee email availability during outages?

Microsoft Defender does not guarantee email availability during outages because it runs inside the Microsoft 365 environment. If Microsoft goes down, so do inboxes and Defender protections.

Think about when a customer sends an urgent purchase order during a Microsoft outage. The message never reaches your team. By the time service is restored, the opportunity is lost.

In Qualysec’s analysis, they found the average cost of a small business cyber incident is around $120,000, factoring in lost revenue, downtime, and recovery costs. Even a short email outage can have a direct financial impact. 

Perimeter continuity can also journal and reinject messages back into Microsoft 365 once service returns, preserving records and conversation history.

Mimecast provides always-on email continuity outside Microsoft 365, allowing employees to send and receive messages as usual—even when Microsoft services are unavailable.

Do email gateways simplify compliance and encryption management?

Yes, email gateways simplify compliance and encryption by providing centralized, easy-to-manage controls across all email. Microsoft Defender includes some compliance features, but settings are fragmented and require specialized expertise.

Key compliance needs SMBs face:

• Retention: Proving data was preserved for audits.
• Search: Quickly finding historical email for investigations.
• Encryption: Securing sensitive data with minimal IT effort.

And as many SMBs know, regulatory fines are steep. GDPR penalties can reach €20 million or 4% of annual revenue, whichever is higher.

Policy engines can auto-apply encryption and retention by role or data type, so departments like finance and HR get the right protections by default.

Mimecast strengthens Defender by delivering automated encryption, searchable archiving, and audit-ready reporting. Together, they help SMBs meet regulatory demands without overwhelming IT teams.

How do gateways support hybrid or multi-cloud environments where Microsoft isn’t the only provider?

Email security gateways support hybrid and multi-cloud environments by securing traffic across diverse systems, while Microsoft Defender is limited to Microsoft 365.

Other common questions:

• What if we still run legacy mail servers? → Defender doesn’t cover them.
• What if we use multiple providers? → Defender only protects Microsoft.
• What if we’re migrating to the cloud? → Gateways secure both old and new systems during transition.

A common policy and logging layer across providers also streamlines audits and accelerates investigations.

Mimecast ensures consistent protection across Microsoft 365, legacy servers, and third-party platforms, providing coverage throughout hybrid and multi-cloud environments.

When does it make sense to use both Microsoft Defender and an email security gateway?

It makes sense to use both Microsoft Defender and an email security gateway when businesses want defense-in-depth that reduces blind spots and ensures resilience. Defender covers internal tenant protections, while gateways strengthen the perimeter.

Together, the two create a layered system that:

• Blocks threats earlier in the flow
• Simplifies compliance through centralized controls
• Keeps email available during outages

This pairing is especially valuable during migrations or mergers when mail routes change frequently and consistency is critical.

Mimecast integrates directly with Microsoft 365, complementing Defender with perimeter filtering, continuity, and compliance. The result is layered security that adapts to modern threats without adding unnecessary complexity.

Conclusion: Microsoft Defender + Mimecast = Email Security Without Blind Spots

Microsoft Defender delivers strong native protection, but it cannot fully replace an email security gateway. Gaps in timing, spam control, compliance, continuity, and hybrid coverage leave organizations exposed if they rely on Defender alone.

Mimecast fills those gaps with perimeter protection, advanced controls, and resilience. Together, Microsoft Defender and Mimecast deliver email security without blind spots—protecting employees, customers, and data while keeping communication flowing.

Ready to secure your email without blind spots? See how Mimecast’s Secure Email Gateway works alongside Microsoft Defender to block threats, ensure compliance, and keep communication flowing. Get your free M365 Threat Scan today →