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What is a secure email gateway?

SEG definition

A secure email gateway (SEG) or a secure email server (SEC) is a type of email security software that sits between inbound and outbound email communication. Every email that is sent to and from an organization passes through this gateway to ensure that its contents are not malicious or a sign of a data leak. It prevents unwanted emails in user inboxes like spam, phishing emails, emails containing malware, and more. In many ways email gateways are the first line of defense for email security. 

A timeline infographic illustrating different levels of email cyber attacks and security coverage.

How does SEG work?

Secure Email Gateways (SEGs) are crucial in defending against various email-borne threats. They offer several main features to enhance email security, including sandboxing, Content Disarm and Reconstruction (CDR), Data Loss Prevention (DLP), anti-phishing measures, and post-delivery protection. Here’s a detailed look at each of these features:

1. Sandboxing

Sandboxing involves executing suspicious email attachments or links in a controlled, isolated environment to observe their behavior. This helps in detecting zero-day exploits and sophisticated malware that traditional signature-based defenses might miss. The sandbox mimics a real user environment to analyze the potential impact without risking the actual network.

2. Content Disarm and Reconstruction (CDR)

CDR is a proactive security measure that focuses on the content within the emails. It involves analyzing and breaking down the content to identify and remove any potentially malicious code. The clean content is then reconstructed into a safe version before delivery. Unlike traditional detection methods, CDR doesn’t rely on detecting known threats but instead ensures that all delivered content is safe.

3. Data Loss Prevention (DLP)

DLP systems are designed to prevent sensitive information from being sent outside the organization unintentionally or maliciously. SEG DLP features scan outgoing emails for predefined patterns that match sensitive data, such as personal information, financial details, or proprietary information. If such data is detected, the email can be blocked, quarantined, or encrypted before sending.

4. Anti-Phishing

Anti-phishing mechanisms in SEGs are essential for protecting users from deceptive emails designed to steal sensitive information like login credentials or financial information. These features typically include:

  • URL scanning: Checking links within emails against databases of known phishing sites.
  • Machine learning algorithms: Analyzing email content for characteristics typical of phishing attempts.
  • User awareness: Flagging suspicious emails to alert recipients or automatically quarantining them.

5. Post-Delivery Protection

Post-delivery protection addresses threats that are identified after an email has been delivered to the user’s inbox. This includes:

  • Retrospective analysis: Re-scanning emails as new threat intelligence becomes available.
  • Automated remediation: Removing or quarantining emails that are identified as malicious after delivery.
  • User reporting mechanisms: Allowing users to report suspicious emails, which are then analyzed, and necessary actions are taken to mitigate the risk.

What security threats can SEG address?

SEG can be useful against the following threats:  

1. Phishing

Phishing involves fraudulent attempts to obtain sensitive information by disguising as a trustworthy entity in electronic communication. SEGs protect against phishing by:

  • Scanning email content and links for phishing indicators.
  • Using machine learning algorithms to identify phishing patterns.
  • Blocking or quarantining suspected phishing emails.

2. Spear Phishing

Spear phishing is a more targeted form of phishing where attackers tailor their messages to a specific individual or organization. SEGs combat spear phishing by:

  • Analyzing email headers, content, and sender behavior.
  • Detecting anomalies that suggest a spear phishing attempt.
  • Employing advanced threat intelligence to identify and block such targeted attacks.

3. Malware and Ransomware

These threats involve malicious software that can infect a system, encrypt files, and demand a ransom. SEGs address malware and ransomware by:

  • Scanning attachments and links for known malware signatures.
  • Using sandboxing to execute and analyze suspicious attachments in a controlled environment.
  • Applying Content Disarm and Reconstruction (CDR) to neutralize potentially malicious code.

4. Spam

Spam refers to unsolicited bulk emails that can clutter inboxes and potentially contain malicious links or attachments. SEGs manage spam by:

  • Employing robust spam filters that use heuristics, blacklists, and content analysis.
  • Continuously updating spam detection algorithms to adapt to new spam techniques.
  • Ensuring legitimate emails are not falsely flagged as spam (reducing false positives).

5. Business Email Compromise (BEC)

BEC is a type of phishing attack where attackers impersonate business executives to trick employees into transferring money or disclosing sensitive information. SEGs protect against BEC by:

  • Analyzing email content and context to identify impersonation attempts.
  • Implementing policy-based controls to flag or block emails that violate organizational norms.
  • Utilizing machine learning to recognize and alert on anomalous communication patterns.

6. Data Leakage

Data leakage involves the unauthorized transmission of sensitive information outside the organization. SEGs prevent data leakage by:

  • Implementing Data Loss Prevention (DLP) features to scan outgoing emails for sensitive information.
  • Blocking, quarantining, or encrypting emails that contain sensitive data.
  • Enforcing policies to ensure compliance with data protection regulations.

7. Zero-Day Exploits

Zero-day exploits are attacks that exploit previously unknown vulnerabilities. SEGs defend against zero-day exploits by:

  • Using sandboxing to detect and analyze unknown threats in a controlled environment.
  • Applying heuristic and behavioral analysis to identify suspicious activity.
  • Updating threat intelligence continuously to adapt to emerging threats.

8. Email Spoofing

Email spoofing involves forging the sender's address to make an email appear as if it is from a legitimate source. SEGs prevent spoofing by:

  • Implementing authentication protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance).
  • Verifying the sender's authenticity before delivering emails to the recipient.

9. Account Takeover

Account takeover occurs when attackers gain unauthorized access to email accounts. SEGs mitigate this threat by:

  • Monitoring for unusual login attempts and access patterns.
  • Implementing multi-factor authentication (MFA) to secure email accounts.
  • Alerting administrators and users to suspicious activities.

10. Advanced Persistent Threats (APTs)

APTs are prolonged and targeted cyberattacks aimed at stealing data or surveilling a specific organization. SEGs counter APTs by:

  • Continuously monitoring email traffic for signs of APT tactics, techniques, and procedures.
  • Employing advanced analytics and threat intelligence to detect long-term, low-and-slow attacks.
  • Coordinating with other security tools to provide comprehensive defense.

SEG challenges

While Secure Email Gateways (SEGs) are essential for protecting against a range of email-borne threats, they do have some limitations when it comes to detecting sophisticated threats. Here are some of the main drawbacks:

1. Evasion Techniques

Sophisticated attackers often employ advanced evasion techniques to bypass SEGs:

  • Polymorphic Malware: Malware that changes its code to avoid signature detection.
  • Obfuscation: Using encryption or encoding to hide malicious payloads.
  • Zero-Day Exploits: New vulnerabilities that SEGs may not yet recognize.

2. Delayed Detection

SEGs may not always immediately detect sophisticated threats:

  • Post-Delivery Threats: Some malicious content may only become active or detectable after delivery, requiring post-delivery protection mechanisms.
  • Retrospective Analysis: Advanced threats may require continuous monitoring and later analysis, which can delay response times.

3. False Positives and Negatives

The balance between security and usability can be challenging:

  • False Positives: Legitimate emails may be flagged as malicious, disrupting business operations.
  • False Negatives: Sophisticated threats may occasionally slip through undetected, especially if they closely mimic legitimate traffic.

4. Resource Intensive

Sophisticated threat detection often requires substantial computational resources:

  • Sandboxing: Executing and analyzing attachments in a sandbox environment is resource-intensive and may not be scalable for large volumes of emails.
  • Heuristic and Behavioral Analysis: These methods require significant processing power and may introduce latency.

5. Dependency on Threat Intelligence

SEGs rely heavily on up-to-date threat intelligence:

  • Lag in Updates: There can be a lag between the emergence of new threats and the updating of SEG databases and algorithms.
  • Threat Intelligence Gaps: Incomplete or outdated threat intelligence can result in missed detections.

6. Limited Scope of DLP

Data Loss Prevention (DLP) capabilities in SEGs can be limited:

  • Pattern Matching: DLP often relies on predefined patterns and rules, which sophisticated threats can sometimes circumvent.
  • Contextual Analysis: Understanding the context of data to prevent leakage is complex and not always accurately handled by SEGs.

7. Complex Configurations

Ensuring effective threat detection requires complex configurations:

  • Policy Management: Setting up and maintaining effective security policies can be complex and time-consuming.
  • Integration Challenges: SEGs need to be effectively integrated with other security tools and systems to provide comprehensive protection, which can be challenging.

8. Human Factor

User behavior can undermine SEG effectiveness:

  • Social Engineering: Highly targeted social engineering attacks (e.g., spear phishing) can trick even well-trained users, potentially bypassing SEG defenses.
  • User Compliance: The effectiveness of SEGs can be compromised if users do not adhere to security policies or report suspicious emails promptly.

9. Encrypted Email Traffic

Encrypted emails pose a challenge for SEGs:

  • SSL/TLS Inspection: Inspecting encrypted traffic requires SEGs to decrypt and re-encrypt emails, which can impact performance and privacy.
  • End-to-End Encryption: Emails encrypted end-to-end cannot be inspected by SEGs, potentially allowing threats to pass through undetected.

10. Adaptive Threats

Attackers constantly adapt to security measures:

  • Evolving Tactics: Sophisticated attackers continuously develop new techniques to evade detection, requiring SEGs to be equally adaptive and agile.
  • Advanced Persistent Threats (APTs): SEGs may struggle with detecting and mitigating APTs due to their stealthy and prolonged nature.

While SEGs are a crucial component of email security, they are not infallible and can be challenged by sophisticated threats. Organizations need to employ a multi-layered security strategy, combining SEGs with other security measures like endpoint protection, user training, and threat intelligence to enhance their overall security posture.

The Difference Between SEG and ICES


A secure email gateway (SEG) or a secure email server (SEC) is a type of email security software that sits between inbound and outbound email communication. Every email that is sent to and from an organization passes through this gateway to ensure that its contents are not malicious or a sign of a data leak. It prevents unwanted emails in user inboxes like spam, phishing emails, emails containing malware, and more. In many ways email gateways are the first line of defense for email security. 


The difference between ICES and SEG is that ICES solutions provide protection for cloud environments that can be on-premise or hybrid. ICES uses machine learning and natural language processing (NLP), and connects via API to understand an organizations email activity and protect against advanced phishing attacks. Unlike SEGs, which use a database of known threats, ICES has the capability to identify never before seen threats and socially engineered phishing emails. 

Email Security Vendors: Darktace’s Approach to Email Security

Darktrace’s AI email security uses artificial intelligence and machine learning algorithms to prevent, detect, respond to, and heal from email attacks.

Through its unique understanding of you, rather than knowledge of past attacks, Darktrace/Email stops the most sophisticated and evolving email security risks like generative Al attacks, BEC, account takeover, human error, and ransomware.

In a Self-Learning AI model, the AI has the ability to understand the business from the inside out. That way when activity within the business deviates from ‘normal', the AI can identify this behavior and alert the security team. 

AI can also use real-time data to identify and respond to threats quickly, minimizing the potential damage and saving time for security teams who usually have to parse through a high number of flagged emails. 

One of the key benefits of integrated cloud email security and AI email security is that it can detect threats that may go unnoticed by traditional security systems, which often rely on pre-defined rules and patterns to identify threats. With AI, email security can continuously learn and adapt, providing more comprehensive protection against previously unknown email-based attacks.