What is an Email Firewall?

In this blog, learn what an email firewall is, how it works, how it affects your email deliverability, and how to avoid it.
If you've ever encountered an unexpected email delivery slow-down, been unable to diagnose seemingly impossible email problems, or received distressed emails from clients saying they didn't receive key communications – chances are your business could benefit from understanding the concept of an Email Firewall.

An Email Firewall protects copmanies and users against malicious attacks while ensuring that essential messages reach their desired destinations.

In this blog post, we will explore what exactly an email firewall is, how it works, and why every company should have one in place for optimal deliverability results.

Let's get started!

Definition of Email Firewall

Email firewalls are essential components of network security that safeguard organizations from malicious or unwanted email traffic. They serve as the first line of defense against various email-based threats, including spam, phishing attempts, malware, and other forms of cyberattacks.

These firewalls use techniques and protocols to scrutinize incoming and outgoing emails, filtering out potentially harmful or unsolicited content.

Many of these applications also provide features that scan inbound emails associated with multiple accounts from different domains and have customizable tools that permit users to block certain domains.

Sometimes, emails get sent to spam folders instead, and users can recover them from there.

However, this is not an ideal scenario for salespeople or marketing specialists sending out several campaigns.

How Do Email Firewalls Work?

Sender Verification

Email firewalls employ sender verification techniques to validate the authenticity of the sender's identity. This involves checking the sender's domain against established protocols such as SPF, DKIM, and DMARC.

SPF verifies if the sending server is authorized to send emails on behalf of the domain. DKIM uses cryptographic signatures to verify the integrity of the email's content and the sender's authenticity. DMARC combines SPF and DKIM, providing policies for email handling if authentication fails. By confirming the sender's legitimacy, email firewalls can filter out emails from spoofed or unauthorized sources.

Content Filtering

Content filtering is a crucial aspect of email firewalls that involves analyzing the content of incoming emails to identify spam, malware, or phishing attempts. Various techniques include keyword matching, heuristic analysis, and machine learning algorithms.

Keyword matching involves checking email content against a predefined list of words or phrases commonly found in spam or malicious emails. The heuristic analysis utilizes algorithms to identify suspicious patterns or behaviors that might indicate the presence of spam or malware.

Machine learning algorithms analyze large datasets to learn from known spam or malware patterns and improve detection accuracy over time. Content filtering helps identify and block potentially harmful emails, preventing them from reaching users' inboxes.

Anti-Spam Measures

Email firewalls implement anti-spam measures to combat the ever-present threat of unsolicited bulk emails. These measures include the use of real-time blacklists (RBLs), which maintain lists of known spam sources, IP addresses, or domains with poor email practices.

By checking incoming emails against these blacklists, firewalls can block or flag emails originating from suspicious sources. Bayesian filtering is another anti-spam technique that utilizes statistical algorithms to learn from patterns in legitimate and spam emails. Collaborative filtering combines the efforts of multiple users to identify spam patterns collectively.

These anti-spam measures contribute to reducing the amount of unwanted email traffic and the risk of users falling victim to spam-related threats.

Malware Detection

Email firewalls incorporate antivirus and anti-malware scanners to detect and prevent the distribution of malicious attachments or links. These scanners examine incoming email attachments for known malware signatures, comparing them against an extensive database of identified threats.

Additionally, firewalls use behavioral analysis to identify suspicious behavior within attachments or links that might indicate the presence of previously unknown or zero-day malware.

Advanced firewalls also utilize sandboxing techniques, isolating suspicious attachments in a secure environment to observe their behavior and determine if they pose a threat.

URL Analysis

Email firewalls perform URL analysis to identify and block phishing attempts. Phishing emails often include deceptive links that redirect users to malicious websites.

They do so by comparing embedded URLs against known lists of malicious websites or analyzing them in real-time using link scanning services. These services assess the website's content during user interaction, providing an additional layer of protection against evolving threats.
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How do Email Firewalls Affect Email Deliverability?

Your emails continuously being marked as spam by even just one user will begin to have ripple effects when the firewalls communicate.

The more often you land in spam, the higher the chances of permanently damaging your senders’ reputation, which you do not want.

Slowly, your email address or domain becomes part of a blacklist of problem senders, which may impact your cold email deliverability for other emails not part of this particular firewall network.

Next thing you know, your next email deliverability test has you hitting spam for G-Suite and with deficient numbers for your senders’ reputation.

Climbing back up from this takes time you typically don’t have in sales or marketing, so the best thing to do is to avoid the issue entirely.

You can avoid it entirely by warming up your email. Here's our email warm-up guide to help you get started.

How to Avoid the Email Firewall?

how to avoid the email firewall

Maintain Your Sending Lists Clean & Updated

Make sure that your lists are clean and as up-to-date as possible.

Take some time to look through the lists biweekly to make sure people are still where they are in the correct role.

If they have changed and your email bounces or their new role no longer applies to your product or service, you want to remove them as soon as possible.

Do not give them any reason to mark you as spam or lead you to an email bounce.

Domains or email addresses sending emails out to addresses that often bounce is information that servers and email firewalls can share with each other.

Spammers will send to several addresses at once and do not care to keep their lists clean, as they are focused on numbers.

You do, this because you want to reach actually interested parties.

Demonstrate it to email firewalls by keeping your sending lists updated and as clean as possible.

Avoid Spam-Trigger Words, Links or Images

Anything that may trigger an additional spam filter or that your prospects may read as spam should be avoided.

No spam-trigger words such as a free trial, special characters, or text formatting. For spam filters and people, these are indicators of spam or malware.

Additionally, avoid using images, attachments, or hyperlinks anywhere in the text.

It may be tempting to send out white papers or sign-up links. Still, when you’re running numerous campaigns, it ends up being more detrimental to your deliverability than anything else. Only start sending additional content once you’ve ensured their responsiveness and interest. Anything before that will be seen negatively.

Signatures should also be as clean as possible so your prospects are more likely to receive the email in their inbox.

An excellent way to do this is to create a separate address to send out emails. Keep your official signature and email for internal issues or existing clients separate from the email you use to send out campaigns.

Provide Value

Because this is heavily based on your prospect's opinion of your email being spam or not, you need to ensure that you provide value with every email you send out.

Follow best practices for cold emailing:

  • Have an intriguing subject line.
  • Write a personalized introduction.
  • Keep it short, sweet, and to the point.
  • Demonstrate your knowledge of your prospect and their pain point.
  • Explain how your product or service works and the solution to their problem.
  • Have a clear call to action.

Do not overdo it with word count or come across as too sales-y or heavily personalized. You don’t want your prospect to feel stalked, just recognized.

Remember that your goal at this stage is to educate, improve brand awareness, and network.

Value is what will differentiate you from spam emails.

Ensure Your Target Audience is On-Point

And a great way to make sure your messaging provides value is to ensure your target audience is correct.

You check the emails biweekly, make sure their roles remain the same and that they still represent the audience that fits your ideal customer profile (ICP).

The best way to get sent to spam is to email people that do not need your product or service.

Avoid that entirely by testing and confirming you’re always reaching out to the right people.

And in conjunction with this, secure you segment your emails and messaging accordingly.

Roles, industries, and responsibilities are all different enough that you can employ different approaches. Whether by messaging or outreach style, different types of messaging will provide valuable insight into the customer’s mindset and inform your future steps.

It can open new verticals too, and serve as a way to network for another opportunity!

Warm-Up Your Emails

Having a high engagement rate is a key factor in maintaining your senders’ reputation.

For that, you require prospects to be responsive. And you ensure responsiveness by lowing the steps outlined previously.

However, you are probably sending out many more emails than you will ever receive responses from. This will make email firewalls and similar systems read you as a spammer.

How to avoid it?

Using a warm-up tool such as Unfiltered increases your engagement rate.

The tool artificially creates conversations between your email address and others that go on for several chains. And it creates new ones.

Manual seeding is excellent, but with Warm, your initial email warm-up process for a new domain, address or reviving an older one becomes significantly easier.


Email firewalls may seem complex and foreign to many, but it's essential to know them as they exist as a form of protection against malicious email.

Taking steps to protect your email is relatively simple; encryption, two-factor authentication, and filtering are just a few of the measures you can take.

However, if you're looking for an all-in-one solution, worry no more--Unfiltered provides easy setup and full protection against email hijacking.

With the right safeguards in place, on top of utilizing Unfiltered, you can fully protect even your most sensitive data.
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