Why Secure Email Gateways Fail (and What You Can Do About It)

Email phishing is now the #1 ransomware attack vector—and if you’re responsible for managing a Secure Email Gateway (SEG), you know the frustration of seeing malicious emails bypass filters despite your best efforts. 

You’re not alone. 

Many organizations deploy industry-leading SEGs, yet they still struggle with phishing and ransomware delivery attempts slipping through. Why? Because email security isn’t a set-it-and-forget-it solution—it requires constant testing, fine-tuning, and adaptation to evolving threats. 

Let’s break down why SEGs miss threats, what you can do to strengthen your defenses, and how continuous ransomware defense validation can prove whether your SEG is actually working as intended. 

Why SEGs Fail to Catch Malicious Emails

Attackers Understand SEG Evasion Tactics 

Today’s phishing emails aren’t the sloppy, typo-ridden scams of the past. Threat actors continuously test their payloads against multiple SEGs to find what gets through. Some of their most effective evasion techniques include: 

• Thread Hijacking: Replying to existing email conversations to appear legitimate. 

• Malicious Links Instead of Attachments: Many SEGs are better at blocking infected files than they are at detecting links that lead to malware or phishing sites. 

• Zero-Day Phishing: Attackers deploy unique phishing URLs that are unknown to threat intelligence feeds, allowing them to evade domain reputation checks. 

• Text-Based Social Engineering: Many attacks don’t contain malicious links or attachments at all—just well-crafted deception that tricks employees into transferring funds or sharing credentials. 

Poor SEG Tuning and Configuration Gaps 

Many organizations deploy SEGs with default settings, assuming their vendor’s out-of-the-box policies will protect them. But every environment is different, and failure to fine-tune filters leads to missed threats. Common configuration issues include: 

• Over-Reliance on Default Allow Lists: Attackers exploit trusted domains and vendors to bypass SEG rules. 

• Ineffective Keyword-Based Filtering: Phishing emails use obfuscation tactics (e.g., “1nvo1ce” instead of “Invoice”) to evade basic keyword detection. 

• Lack of Dynamic Analysis for Embedded Links: Many SEGs don’t actively analyze what happens after a user clicks on a link.  

• Macro-enabled attachments: Malware loves macros, so why are some SEGs still letting them in? 

• Outdated threat intelligence: If your SEG is only blocking yesterday’s threats, today’s attackers are winning. 

• Shaky authentication policies: No DMARC, DKIM, or SPF? You’re basically handing out VIP passes to spoofed emails. 

• Quarantine chaos: If users can release sketchy emails on their own, your SEG might as well not exist. 

Lack of Continuous Testing and Validation 

A SEG is only as good as its last real-world test. Many organizations assume their SEG is working—until a ransomware attack proves otherwise. The only way to know for sure is through continuous validation that simulates real-world phishing and malware delivery attempts to test whether your SEG is actually blocking what it should. 

In a recent webinar, Reimagining Ransomware Defense: Revealing and Removing the Hidden Risks of Security Control Failures, OnDefend CEO Chris Freedman discussed why SEGs commonly fail. Here’s a short clip:

How to Strengthen Your SEG and Stop More Threats 

1. Continuously Test Your SEG Against Real-World Threats 

The best way to identify weaknesses is to actively test your SEG with the same techniques attackers use. OnDefend’s Ransomware Defense Validation (RDV) includes comprehensive SEG testing, helping organizations: 

• Simulate real-world phishing and malware delivery attempts to see what bypasses the SEG. 
• Test against evolving SEG evasion tactics (thread hijacking, payload-less attacks, dynamic phishing sites). 
• Get detailed reports on misconfigurations, tuning gaps, and bypassed threats. 

2. Fine-Tune Your SEG for Maximum Protection 

Once you identify the gaps, it’s time to optimize your SEG configuration: 

• Enable Link Sandboxing & Time-of-Click Analysis – Ensure your SEG analyzes URLs at the moment the user clicks, not just during initial delivery. 
• Tighten Allow Lists – Remove unnecessary domain exceptions and use behavior-based trust instead of static domain reputation lists. 
• Implement AI-Driven Detection Rules – Some advanced SEGs allow machine learning-based anomaly detection instead of simple static rules. 
• Regularly Update SEG Rules & Policies – Test and adjust based on the latest phishing and malware delivery trends. 

3. Reassess Your SEG Vendor If Necessary 

If your SEG consistently fails real-world attack simulations, it may be time to evaluate other vendors. Not all SEGs offer equal protection, and some provide better: 

• Machine learning-driven phishing detection 
• Advanced BEC protection (analyzing writing styles & context) 
• Deeper integration with EDR/XDR solutions 

The Bottom Line: Test, Tune, and Validate Your SEG Regularly

Email is still the cybercriminal’s favorite playground, and your Secure Email Gateway (SEG) is supposed to be the bouncer at the door. But let’s be real—how often do you check if that bouncer is actually stopping the bad guys?  

Your SEG isn’t magic—it’s only as good as the rules and configuration you set and the tests you run. Don’t assume it’s got you covered just because it’s there. It’s definitely not a set-it-and-forget-it security control. Get hands-on, put it through the wringer, and make sure it’s actually keeping your inbox safe. Because when it comes to email security, guessing isn’t good enough. 

Or better yet, let OnDefend do it for you with real-world phishing and malware testing. SEG testing is inexpensive and a super small lift on your end (just set up an inbox for our tester). Once we have access to the email, we send hundreds of malicious payloads your way. We’ll assess for the risks of spoofing, non-repudiation, and as a threat vector, and after a 7-day testing period, we’ll provide a full report. Get in touch with OnDefend today to get started.

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Resources: https://blog.knowbe4.com/heads-up-email-phishing-is-now-the-top-ransomware-attack-vector