Spam Filter: What Is It?

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Harsha Kiran
Written by
Harsha Kiran

Updated · Nov 17, 2023

Harsha Kiran
Founder | Joined March 2023 | LinkedIn
Harsha Kiran

Harsha Kiran is the founder and innovator of He started it as a personal passion proje... | See full bio

Lorie Tonogbanua
Edited by
Lorie Tonogbanua


Lorie Tonogbanua
Joined June 2023 | LinkedIn
Lorie Tonogbanua

Lorie is an English Language and Literature graduate passionate about writing, research, and learnin... | See full bio

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A spam filter is a program designed to determine whether an email is virus-infected. Then, it prevents the email from infiltrating a user's inbox. 

According to recent spam statistics, around 49% of emails were categorized as spam in 2022. This has prompted internet service providers to use spam filters to prevent the proliferation of junk emails.

Email spam filtering is essential, especially for businesses. In this article, you’ll learn what a spam filter is and how it works. Read on to learn how you can utilize it for your own business.

What is Spam Filtering?

Approximately 60 billion spam emails were sent daily from 2019 to 2023. In response, service providers have been constantly developing spam filters to protect their clients.

Spam filtering is detecting whether an email is harmful or not. It’s necessary for businesses because it can help eliminate the risk of viruses, phishing attacks, and other malicious content.

Even though some spam emails are harmless, they can complicate your inbox management. Understanding how spam filtering works is important to choose the right filter and to organize your inbox according to your needs.

Understanding How a Spam Filter Works

A spam filter classifies emails according to 3 criteria

Cybercriminals can use spam email to distribute kinds of malware or conduct phishing attacks. In 2022, 500 million phishing incidents were reported. Many of them involved large data breaches totaling $50 million in losses.

To combat this, spam filters apply specialized screening techniques to determine an email's source or content. This is how they can recognize phishing or marketing emails and flag legitimate ones.

There are three key areas that spam filters or criteria look at:

  • The content of the email,
  • The legitimacy of the sender, and
  • The interaction between the sender and the email.

Incoming emails enter through a “filter” and get sorted depending on the criteria. Spam filters use the heuristics method, where each email is subjected to hundreds of predefined rules.

Each rule rates the likelihood that a message is spam. If the score exceeds a predetermined level, the email is marked as spam and prevented from being forwarded.

When the filter flags an email as spam, it can remove the spam email immediately or after a certain time. It can also place the email into a Spam Folder. 

💡 Did You Know?

Email marketing, in itself, is not spam. Businesses prefer to use it in spreading content because email marketing has a 3,600% return on investment (ROI). The promising returns attract businesses that don’t know how to use email marketing, resulting in spam.

3 Categories of Spam Detection

By 2025, the number of daily emails will rise to 376.4 billion, and spam will also grow along those figures. Because large-scale emailing is cheaper than video ads, and the profit potential is substantial, email spamming will only continue to surge.

Even with the influx of spam, three different categories of spam detection are used to manage your emails. These categories are based on where they are in the email-sending process. 

Here’s a list of spam detections based on where they are located within the email-sending process:

Gateway Spam Filters

A gateway spam filter behind a network firewall scans incoming emails for possible threats. The mail must pass through the ‘security gate’ before reaching the company's network.

Hosted Spam Filters

Unlike a gateway spam filter, a hosted spam filter is located within a cloud. It’s active before the email enters and is used inside a network. 

One advantage of spam filters housed in the cloud is that they can be promptly updated with the newest software. Consumers always have the most recent product available to detect the newest spam trends. 

Desktop Spam Filters

Users install a desktop spam filter by downloading them locally. One advantage of these filters is that users can customize them to provide the best filtration methods unique to their requirements.

Understanding the uniqueness of each spam filter can help you choose the one that will work best for your needs. 

✅ Pro Tip:

If you’re looking for spam filters, try Mailwasher, ESET Protect Mail Plus, Trustifi Inbound Shield, Hornetsecurity Email Spam Filter and Malware Protection Service, and SpamTitan. They’re five of the best anti-spam software and blockers in 2023, according to Comparitech.

Types of Spam Filters

While the filter category speaks more to where it exists, the types of spam filters speak more to what the filter is looking for.

These types of spam filters also have different methods of filtering spam emails. 

Here are the different types of spam filters: 

Content Filters

Content filters look into the body of the email. The filter checks for suspicious content like trigger words or images that spammers usually use.

Content filtering technology also screens and restricts unwanted websites, search engines, DNS, and proxy content. 

Blacklist Filters

Blacklist filters block emails from senders included in a list of spammers, which is regularly updated because spammers can quickly switch email addresses.

A business can safeguard its interests by using its blacklist spam filtering. Companies use it against a target headhunter who wants to take their employees. They also leverage it to stop sales offers and promotions, eliminating work distractions.

Header Filters

Header filters scan the email’s header information to see if the sender is not blacklisted or look for any suspicious stops the email may have encountered before reaching the recipient.

✅ Pro Tip:

Email filtering is part of any parental control software. To protect your kids from potentially dangerous emails and content on the Internet, many efficient parental control apps in 2023 are available online.

Language Filters

Users may need to check their spam folders when anticipating emails from another language. The language filter classifies a legitimate email from a business connection or a customer from another nation as spam.

Rule-Based Filters

Rule-based filters enable the filtration of certain emails according to predetermined standards. Say the user blocks messages from a specific sender or emails with particular words in the subject line or header; it operates to filter those based on the rules set by the user.

Bayesian Filter

Bayesian Filter learns your preferences for spam. When the user designates an email as spam, the system takes note of its traits and searches for others in incoming emails that share them, screening anything that matches the formula as spam.

👍 Helpful Articles:

Spam calls are more direct since the spammer speaks to you in real time. Unlike emails, this situation can be frightening because you may be talking to a potential cybercriminal. To know more about this, here are helpful articles that will shed light on spam calls:

Bottom Line

Unwanted emails are annoying, especially when they make finding important messages challenging. However, since spam emails usually have phishing and malware links, it becomes a major threat to your device or computer and all your accounts and contacts.

Luckily, technology has allowed users to filter inboxes to avoid getting clogged up by spam emails. Email providers now include features that detect spam from the inbox to add security to their users.

While technology provides tools for cyber security, human errors are still a huge factor in falling victim to cyberattacks. As they always say, the best method is to think before you click.


What do spam filters look for?

Spam filters scan email bodies and headers for suspicious information. They also screen content that doesn’t comply with admin-configured predefined rules or is written in foreign languages.

How do I get a spam filter?

Well-known email services have spam filters. Gmail, for example, blocks almost 100 million daily spam emails. You can also inquire about the filters with your email service provider.

Why do spam filters exist?

This tool exists to protect users' interests by filtering out malicious emails. It prevents malware from entering a system and infecting users’ PCs.


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