Whether deployed as a cloud service or on-premises, email security—and especially email security gateways—is of growing importance to organizations of all sizes, particularly given the upheavals to the working world over the last several years.

It is estimated that in the post-pandemic world, one quarter of the workforce will choose to permanently work remote. In some sectors, such as technology and finance, that figure could reach 40%. This new reality opens organizations up to myriad vulnerabilities as a hybrid workforce connects to your servers from unsecured devices on unsecured private networks. Email security will be more important than ever.

When it comes to solutions for protecting users from malicious emails, email gateways are one of the most popular traditional security tools, but what do they do exactly? An email gateway is a type of email server that protects an enterprise network by monitoring all ingoing and outgoing emails to employee accounts.

This article will examine what email security gateways (ESGs) are, how they work, why they’re important for securing your organization’s environment, and some of the limitations you need to know about.

What are Email Security Gateways and What Do They Do?

Email Security Gateways (ESGs) or secure email gateways are a cyber security solution that functions as an email-specific firewall, protecting an organization’s internal mail servers from malicious content trying to gain system access from the public Internet.

ESGs can also be configured to prevent sensitive data from leaving the organization in outgoing emails or to automatically encrypt emails when sensitive information does need to be shared.

How Do Email Security Gateways Work?

ESGs leverage a collection of the following technologies to block email threats and other social engineering attacks and protect systems:

  • Threat intelligence – Provides security analysts with intelligence on what techniques are being used to attack the environment, who is targeting them, and what they can do to remediate it.
  • URL scanning – Real-time scanning and rewriting of URLs to identify and block phishing attempts.
  • Attachment scanning/protection – antivirus scanning with static file analysis and behavioral sandboxing to identify malware attachments.
  • Sandboxing – Inspecting content, and running malicious code in an isolated environment where it has no risk of infecting the system.
  • Post-delivery protection – An API that integrates with an email service to withdraw malicious emails from the user’s inbox that have slipped through the net.

By enforcing specific rules about the kinds of emails that can enter or leave your network, and by analyzing all incoming and outgoing email to an organization, ESGs flag potentially harmful phishing attacks, ransomware, and other forms of malware before they reach an endpoint and start doing damage.

Why are Email Security Gateways Important?

ESGs are crucial to a strong cyber security posture and offer businesses a variety of benefits.

Emails remain one of the most . They offer the opportunity for low-cost, high-reward attacks, particularly since additional vulnerabilities were introduced to many organizations by widespread work-from-home during the pandemic.

The rise in vulnerabilities is one reason why email-based attacks like phishing are at record levels since 2020, according to the FBI.

With the built-in security of many email programs full of shortcomings, tools like ESG are an important way to help secure an organization from email-based attacks. They can scan emails for malicious content, such as links to phishing websites or malicious attachments and block them before they reach the user.

Filtering out these emails is important because it reduces the risk of employees being exposed to convincing phishing scams that might persuade them into downloading a malware attachment that causes a data breach.

As the number of email-based threats increases, you can’t rely on antivirus and anti-spam tools alone to keep these threats at bay, you need a secure email gateway to filter them out, and security awareness training to teach employees how to ignore and report any that slip through your defenses.

ESGs also offer organizations another way to meet compliance needs. With email archiving and encryption capabilities, ESG can help organizations handle sensitive data securely and in compliance with relevant legislation.

Cloud or On-Premises Email Security Gateway? Which is Better?

While ESGs can be cloud-based or deployed on-premises, the choice usually comes down to whether an organization maintains an on-premises email server or whether they are already using a cloud-based email service. Nevertheless, there are differences depending on what kind of ESG you choose to run.

As with any on-premises hardware or software, an organization with an on-premises ESG is responsible for purchasing and maintaining the system in a way that you wouldn’t have to if you were using one hosted in the cloud by a third-party provider. They must also ensure software is updated regularly to maintain peak performance.

Organizations that rely on an ESG in the cloud, on the other hand, have no such responsibilities. Instead, the cyber security provider will ensure the hardware and software the ESG relies on will be properly maintained and updated.

Cloud-based ESGs also offer users the benefit of rapid scalability. Should email traffic spike or the number of users suddenly increase, a cloud-based ESG provider has the extra capacity to shoulder the load and maintain performance in a way that most on-premises ESG hosts do not, whether due to technical or staffing reasons.

Should You Use an Open-Source Email Gateway?

When looking for a secure email gateway you can either purchase a proprietary tool or as a free to use, open-source solution. While open-source and free email security gateways may be convenient and low-cost, they are easy to obtain, and hackers will have tested their tools against them. Therefore, they might not have the same level of threat intelligence and detection capabilities that a proprietary solution has.

Also, free security software and services tend to lag in updating and cannot pivot to combat the latest innovation by malicious actors in the same way that a paid service can.

As a result, it’s often a good idea to deploy a secure email gateway that’s maintained by a reputable provider so there’s less risk of missing manipulation attempts.

The Limitations of Secure Email Gateways

While secure email gateways are very useful tools, they aren’t foolproof. Drawbacks include:

  • Email-only focus: ESGs by design protect email. Yet more and more organizations now rely on cloud-based file sharing and collaboration tools, like Google Drive, Microsoft Teams, or Slack. These services aren’t covered by an ESG and so leave the organization vulnerable.
  • Single-layer security: Because some ESGs disable the built-in security protections of some email providers, an additional layer of protection is removed should something slip by the ESG.
  • Root domain vulnerability: Clever hackers can potentially bypass an ESG by sending emails directly to the root domains of organizations whose DNS is managed by Microsoft or Google.
  • Known-malware signatures: ESGs rely on them to detect threats. While this protects against the majority of threats, it doesn’t defend against zero-day vulnerabilities that don’t match the malware signatures of existing threats.

When considering that most email gateways can only detect threats in the body of emails, it’s important to use antivirus tools alongside secure email gateways to bolster your defenses against an attacker attempting to obfuscate malicious code within an attachment.

And it’s because no cyber security measure is foolproof that it’s crucial to implement effective   alongside measures like ESGs to reduce the risk of a data breach. Proven security awareness training and phishing simulation platforms can keep employees’ phishing and social engineering risks top of mind, boosting your level of human security in case anything slips through to the level of the individual user.


Email Security Gateways (ESGs) offer organizations a way to protect internal mail servers from malicious inbound content while also offering the ability to protect and encrypt sensitive information when it needs to be shared externally, bolstering both security and compliance.

By leveraging a variety of different technologies to protect mail systems, ESGs are a crucial part of a strong, effective cyber security program.

Technological safeguards can’t guarantee information security, however. That’s why by implementing multifaceted awareness training that includes real-world phishing simulations and consistent best practice reinforcement, your organization will be better equipped to detect and avoid common cyber threats.



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