Secure email gateways (SEGs) have long been a cornerstone of enterprise email defense. They inspect incoming and outgoing messages, block known threats, and enforce policies. However, many malicious emails manage to evade these conventional safeguards in today's threat landscape, where attackers employ social engineering, artificial intelligence (AI), and malicious methods.
Security teams around the world are discovering more and more “anomalies” that evade SEGs entirely. They lurk in inboxes, exploiting trust, internal relationships, or identity context, often executing damage before they’re even detected.
Secure Email Gateways (SEGs) were once sufficient to protect organizations from spam, malware, and bulk phishing campaigns at the email perimeter. But over time, attackers have evolved. As shown in Security Boulevard, SEGs increasingly fall short in light of today’s cyber threats. Below are the primary limitations they suffer from:
Attackers no longer send broad “spray and pray” phishing campaigns. Instead, they carefully craft messages to impersonate trusted individuals, mimic internal writing style, and embed references to personal or organizational context. Some leverage AI, such as ChatGPT, to generate highly convincing and context-aware emails. Because these emails often don’t contain obvious “red flags” (suspicious domains, attachments, or links), SEGs struggle to detect them. In fact, one estimate cited in the article is that ~19% of phishing emails go undetected by Microsoft Defender, and this gap widens when the phishing attempt is carefully disguised.
A core weakness of many SEGs is their reliance on traditional techniques, known malicious signatures, global blacklists, IP reputation, and historical threat intelligence. These mechanisms work only as long as threats are already known. But modern attacks increasingly use zero-day malware, novel payloads, or fresh phishing domains that have no prior history. Without contextual or behavioral visibility, SEGs lack the telemetry to detect such advanced threats.
Related: What is a zero-day exploit?
SEGs are often blunt instruments: they inspect messages broadly (by sender, header, domain, and attachments) but lack deep semantic or structural analysis. The Security Boulevard article notes that SEGs typically struggle to dissect macro-laden documents, embedded scripts, polymorphic malware, or deeply nested attachments. Indeed, an estimate cited in the article claims that 14 % of email-related malware evades SEG defenses, not a trivial number considering that email remains one of the primary channels for payload delivery.
SEGs aren’t invulnerable themselves. The article cites a real example: in November 2022, a vulnerability in Cisco’s Secure Email Gateway let attackers bypass filters altogether and deliver malicious payloads. More broadly, SEGs require constant configuration, tuning, whitelist/blacklist updates, and staff oversight, a heavy operational burden. Misconfigurations or stale settings create gaps that threat actors can exploit.
Also, too many false positives can overwhelm IT/security teams, leading to compromises in filtering policies just to reduce noise.
Even if SEGs could become perfect (which they can’t), their scope is limited: they cover only email. Modern attack campaigns often span multiple channels: SMS-based phishing (smishing), voice phishing (vishing), WhatsApp or social media messaging, impersonation, or hybrid approaches. SEGs offer no protection against non-email vectors.
Modern attackers use generative AI, such as GPTs, to craft flawless, personalized emails that imitate internal style, referencing past projects, names, meeting schedules, or shared context. According to Paubox, phishing is the leading cause of healthcare breaches. As of 2024, over 70% of healthcare data breaches originated from phishing attacks.
These phishing attempts may impersonate HR, IT, finance, or leadership, embedding links that point to lookalike domains or staged login forms. The text and metadata are so convincing that neither human recipients nor signature-based filters detect them.
Because a SEG might only look at domain reputation, known phishing URLs, or blacklisted senders, these threats often slip through. It takes behavioral and identity context to catch them.
BEC attacks are among the most insidious because they typically don’t carry malware or malicious links. Instead, they exploit trust and authority. An attacker impersonates an executive or finance officer, often via spoofed “from” addresses or lookalike domains, and instructs a finance team to wire funds or pay invoices.
The email content is often plain text with no attachments or suspicious URLs, so SEGs may not flag it. As Global Security Mag states, “40% of the BEC emails uncovered were AI-generated, and in some instances, AI likely created the entire message.” This can make them indistinguishable from authentic correspondence.
Detecting BEC often requires comparing writing style, tone, and context against historical emails from that sender and flagging deviations, such as sudden urgency, new routing instructions, awkward phrasing, something a rule-based SEG rarely does.
Attackers are increasingly compromising trusted third parties (vendors or suppliers) to send malicious emails, known as vendor email compromise (VEC). Because these emails come from known, legitimate domains, they often bypass SEGs entirely.
In such attacks, the attacker hijacks the vendor’s email environment or spoofs their communications and injects malicious content or requests into existing threads. For example, they might send a payment request that appears to be from your vendor’s finance team. Because the domain is legitimate, trust is high, and SEGs often don’t flag it.
Detecting VEC demands contextual anomaly detection: “Is this vendor email asking for something unusual at this time? Is the tone or behavior consistent with past vendor communications?”
Related: What is a supply chain attack and how can it be prevented?
When an attacker gains access to a legitimate mailbox, they can use it as a launchpad for further phishing or fraud. These internal threats are especially dangerous because they come from a trusted source and often maintain reply threads with previous context.
Such abuse is nearly invisible to SEGs. A message from a known internal address is rarely blocked. The attacker may gradually send malicious emails to internal or external contacts over time, blending in with normal traffic.
Catching this requires continuous monitoring of sending patterns (e.g. volume, timing, recipients) and comparing them against baseline behavior, tasks more suited to user and entity behavior analytics (UEBA) than SEG logic.
Read more: What is account takeover (ATO)?
Many attacks hide malicious code inside attachments such as Word, Excel, or PDF files that contain macros or embedded scripts. Today's malicious attachments often morph every time (polymorphism), or use zero-day exploits that have no known signature. Signature-based SEGs will miss these.
An example: a Word document with an innocuous-looking macro that downloads a payload only after some time, or that takes advantage of a new exploit. Traditional gateways often sandbox attachments, but sophisticated malware may detect the sandbox or delay execution, evading detection.
Modern defenses must combine static + dynamic analysis, machine learning models examining attachment structure, and behavioral sandboxing. Research like “Holmes: An Efficient and Lightweight Semantic Based Anomalous Email Detector” demonstrates systems that convert email content to semantic representations and detect anomalies not caught by standard gateways.
Sometimes the malicious email doesn’t carry any link or attachment; instead, it gives plain instructions. For example:
Because there is no URL or payload to scan, SEGs often have nothing to flag. These instructions exploit trust and human response, not technical vulnerabilities.
Catching such attacks requires semantic and intent analysis; the system must “understand” that the instruction is unusual for this sender at this time.
Attackers can infiltrate existing email threads by responding to a current conversation with harmful content, either through spoofing or account compromise. Because the thread is legitimate, users often accept the new content without suspicion.
Since the email appears to be a continuation of a prior conversation and often lacks new suspicious artifacts, SEGs struggle to detect it. The malicious content is embedded in context, making it subtle.
Not all threats come from external attackers. Sometimes employees or insiders, intentionally or under coercion, send malicious emails. Because their accounts are legitimate and authorized, SEGs generally won’t block them.
For example, a finance manager colluding to redirect funds or a disgruntled employee sending confidential documents to external parties by email. The content may look normal or innocuous enough to pass a SEG.
Preventing insider anomalies often relies on anomaly detection on volume, timing, attachments, or recipients, integrating with data loss prevention (DLP) or UEBA systems that flag unusual internal-to-external traffic.
Read also: Insider threats in healthcare
As attackers evolve and traditional SEGs struggle to keep pace, healthcare and enterprise organizations need a more adaptive, behavior-aware approach to email protection. That’s where Paubox’s Inbound Email Security stands out.
Paubox goes beyond perimeter-based filtering by analyzing inbound messages within a zero-trust framework, ensuring that only verified, contextually safe emails reach your inboxes. Its solution operates natively within your email flow, with no need for portals or extra logins, allowing for seamless protection without disrupting workflows.
Using advanced AI and pattern recognition, Paubox detects anomalies that SEGs often miss—such as tone-mimicking attacks, display name spoofing, and vendor impersonation. It continuously learns from evolving threat behaviors to flag suspicious emails that may look legitimate but deviate from normal communication patterns.
In addition, Paubox integrates with its HIPAA compliant email encryption suite, Paubox Email Suite, ensuring that all inbound and outbound messages remain protected according to HIPAA’s privacy standards. This layered approach combines contextual threat detection, identity validation, and regulatory compliance, a critical advantage for organizations that handle sensitive data like ePHI.
By bridging the gap between trust and technology, Paubox enables security teams to stay ahead of modern threats while maintaining the simplicity and efficiency healthcare and enterprise environments demand.
By adopting behavioral and identity-aware solutions that analyze user patterns, writing styles, and intent. Integrating UEBA, DLP, and AI-driven analytics helps detect anomalies that don’t trigger signature-based rules.
Employees are often the last line of defense. Regular security awareness training and easy reporting tools empower users to flag suspicious messages that even advanced systems might miss.
Experts recommend quarterly red teaming or phishing simulations to evaluate SEG effectiveness, test detection systems, and identify blind spots in real-world conditions.
Yes. SEGs remain useful for blocking spam and known malware. However, they should be part of a multi-layered defense strategy that includes AI-driven anomaly detection, identity protection, and continuous monitoring.