Email Security Gateway Comparison: Proofpoint, Mimecast, Defender

Gateway vs Native Cloud Email Security

The traditional email security gateway — a separate appliance or service that sits in front of your mail server — is being challenged by native cloud security features built into Google Workspace and Microsoft 365. For organisations already on these platforms, the native security controls (Gmail Advanced Protection, Microsoft Defender for Office 365 Plan 2) provide substantial protection without the gateway complexity and its associated deliverability risks.

The deliverability case for native cloud email security is clear: Google and Microsoft's native security tools do not modify messages in ways that break authentication, because they are integrated into the same mail stack that applies authentication. A third-party gateway layered on top of Google Workspace introduces DKIM-breaking risks that Gmail's own security controls do not. For pure inbound protection, native cloud controls are often the better choice. Standalone gateway products add genuine value primarily for on-premises Exchange environments, complex DLP requirements, or multi-cloud deployments where a single control plane is needed.

Testing Your Gateway's Impact on Authentication

The definitive test of whether your gateway is affecting authentication is straightforward: send a test message from your full outbound path to a mail-tester.com address and to a seed address at Gmail. Check the Authentication-Results header in the received message. Every field — SPF, DKIM, and DMARC — should show pass, and the DKIM d= value should match your sending domain, not the gateway's.

If DKIM shows fail or the d= value is the gateway's domain, the gateway is re-signing or breaking DKIM. If SPF shows softfail or fail, the gateway's relay IPs are not in your SPF record. Fix both before considering the gateway deployment complete. Running this test monthly as part of a standard deliverability health check catches any configuration drift before it becomes a deliverability incident.

Total Cost Considerations

Enterprise email gateway pricing is not published — it is negotiated per organisation based on mailbox count and features. As a rough benchmark for 2025: Proofpoint Email Protection starts at approximately €4–8 per mailbox per month for mid-market organisations; Mimecast is broadly similar; Microsoft Defender for Office 365 Plan 2 is included in M365 E5 licences (~€54/user/month) or available as a standalone add-on at ~€4.50/user/month for Plan 2. Barracuda and Cisco IronPort are often lower on per-mailbox cost but may require dedicated hardware amortisation.

For the deliverability decision: the gateway's licensing cost is secondary to the operational cost of the deliverability engineering required to maintain correct authentication through the gateway. Proofpoint and Mimecast both require skilled configuration to preserve DKIM integrity — organisations without an email security specialist typically need professional services at deployment. Factor this into the total cost comparison.

SIEM Integration for Email Security Events

Enterprise gateway deployments typically require integration with a Security Information and Event Management (SIEM) system — Splunk, Microsoft Sentinel, IBM QRadar, or similar. SIEM integration allows email security events (blocked phishing attempts, BEC detections, authentication failures) to be correlated with other security signals across the organisation. All four major gateways support syslog or API-based log export to SIEM platforms.

From a deliverability operations perspective, SIEM integration surfaces one particularly valuable signal: outbound messages blocked by the gateway's policy engine. If legitimate outbound mail is being blocked by a DLP rule or impersonation detection policy, it does not generate bounces — it silently fails to reach the recipient. SIEM logs are often the only place this is visible. Configure an alert for any outbound message blocked by policy that originates from an authenticated internal user — this catches mis-configured policy rules before they generate significant business impact.

How Email Security Gateways Affect Outbound Deliverability

Email security gateways placed in the outbound sending path — between the sending application and the internet — can significantly affect deliverability in ways that are not immediately obvious. Understanding these effects allows infrastructure architects to configure gateways to maximise security without sacrificing deliverability.

The most common gateway deliverability interaction: DKIM signature invalidation. DKIM signatures cover specific message headers and the message body. A security gateway that modifies headers (adding compliance disclaimers, modifying From names, inserting tracking pixels) after the DKIM signature is applied breaks the signature, causing DKIM to fail at receiving servers. The fix: either sign after the gateway (if the gateway provides DKIM signing capability), or configure the gateway to apply its modifications before the DKIM signing step, or exclude the modified headers from the DKIM signed header list (reducing but not eliminating the problem).

The second common interaction: SPF permerror from gateway IP mismatch. If the security gateway's sending IPs are not included in the sending domain's SPF record, messages that transit the gateway will fail SPF (the sending IP — the gateway — is not authorised in the SPF record). Add the gateway's IP ranges to the SPF record, ensuring the record stays below the 10-lookup limit. For large gateway services with many IP ranges expressed as include: mechanisms, the SPF lookup count can quickly exceed 10 — use SPF flattening tools to convert include: mechanisms to their current IP ranges if necessary.

Third interaction: message modification and DMARC alignment. If the security gateway changes the From: header or the MAIL FROM envelope domain without corresponding changes to the DKIM signature, DMARC alignment can fail. Verify DMARC alignment end-to-end (from the final sending IP through the gateway to a test receiving server) by reviewing Authentication-Results headers in a test message, not just by verifying the sending infrastructure configuration before the gateway.