PowerDMARC

What Is a Spam Filter & How It Protects Your Inbox

Yunes Tarada
spam filter

Key Takeaways

  1. Spam filters scan incoming emails to block unwanted or harmful messages.
  2. There are different types of spam filters, including gateway, hosted, desktop, and ISP-based ones.
  3. Using spam filters helps protect against phishing, saves time, reduces malware risk, maintains sender reputation, and improves overall email security.

Every day, around 14.5 billion spam emails are sent globally. That amounts to nearly 45% of all email traffic. Most email services like Gmail or Outlook have a built-in “Spam” or “Junk” folder where such messages are automatically filtered in order to keep them out of your main inbox.

However, although these email spam filters do a decent job, they aren’t perfect. Especially for businesses and high-value targets, basic filtering generally isn’t enough. More advanced threats can still slip through, which is why additional layers of protection are needed.

How Does a Spam Filter Work?

Generally, all spam filters function in the same way: scanning incoming email and determining whether it’s unwanted or malicious. They help shield you from phishing, email spoofing, and other threats that can compromise trust and expose data. Though the concept is simple, the actual mechanisms behind spam detection are varied and increasingly sophisticated.

Most detection methods fall into two broad categories: origin-based techniques and content-based techniques.

Origin-based techniques

Filters that rely on origin-based techniques evaluate where an email is coming from rather than just what’s inside it. They often rely on network information like IP addresses and domain names to judge whether a message is likely to be spam.

One of the most common methods used in this category is a combination of blacklists and whitelists. Blacklists contain lists of email addresses or IPs that are known to send spam emails. If a message comes from a source that is on the blacklist, then the spam filter will likely reject it automatically. Whitelists, on the other hand, help approve messages from known and trusted senders. These lists help guarantee that important messages aren’t mistakenly marked as spam.

A more dynamic version of the blacklist is the Real-Time Blackhole List (RBL). RBLs are maintained by third parties and updated continuously. When an email arrives, the spam filter checks the sender’s IP address against the RBL. If there’s a match, the message is flagged or blocked. Because these lists are managed externally, they require less manual maintenance but also offer less control.

Content-based techniques

Content-based filters analyze what the email actually says and how it’s structured. These techniques are designed to scan the body, subject line, and metadata of emails to determine intent.

Rule-based filters are the most straightforward. They use predefined criteria to flag messages, such as emails that contain common spam phrases like “free money” or “urgent response needed.” Each matching rule carries a score, and when an email exceeds a certain threshold, it’s labeled as spam. However, these filters can be rigid and easy to trick with simple text manipulation.

Bayesian filters offer a more reliable alternative. They use statistical probabilities to decide whether a message is spam or not, based on how frequently certain words appear in known spam versus legitimate emails. These filters also learn over time, becoming more accurate as users interact with their inboxes.

More complex models include Support Vector Machines (SVMs), which use mathematical models to distinguish between spam and non-spam by drawing a boundary between the two categories in a virtual space. These are particularly effective when trained on large datasets.

There are also Artificial Neural Networks (ANNs), which attempt to replicate the way the human brain processes information. By analyzing large amounts of data, they can identify complex patterns in email content that simpler filters might miss. Like Bayesian filters and SVMs, ANNs improve the more they’re trained.

Types of Spam Filters

Spam filters can be deployed at different stages of the email delivery process. It all depends on who manages the emails and where protection is most needed.

Some of the most widely used types of spam filters include:

Gateway spam filters

A gateway spam filter is a type of email security system that screens messages before they reach the internal mail server. In a sense, such a filter is like a security checkpoint that is positioned outside the server, and every email must pass through it in order to be delivered.

When an email arrives, the gateway filter analyzes it based on who sent it, whether the sender’s IP address has been flagged for spam in the past, and whether the content contains suspicious keywords, links, or attachments. It also evaluates the structure of the email to detect common traits of phishing attempts or malware delivery. Many gateway filters allow customization, such as blocking emails from specific countries, domains, or file types.

This kind of filtering is especially useful for companies, schools, and large organizations because it stops threats before they can really do any damage or even touch internal systems. It also keeps the mail server from being overwhelmed with junk messages, saving storage and reducing the risk of system slowdowns.

Hosted spam filters

A hosted spam filter is a cloud-based service used to filter emails before they reach the network. Messages first go to the hosted filter’s servers, where they’re scanned for spam indicators like suspicious links, blacklisted IP addresses, harmful attachments, or abnormal formatting. Then, if they are clean, they get passed through to your inbox, whereas suspicious ones are held back, quarantined, or flagged as spam, depending on the settings.

Unlike gateway filters that require hardware or software installation on-site, hosted spam filters operate off-premises and are managed by third-party providers. This makes them an appealing option for remote teams or organizations without dedicated IT departments.

One of the biggest advantages of hosted spam filters is their scalability. As your organization grows, the service can easily handle more users and messages without requiring you to upgrade hardware or infrastructure.

Desktop spam filters

Desktop spam filters are software programs installed directly onto a user’s personal computer or laptop. Unlike the previous two types of filters that work before emails reach the device, desktop filters scan and sort emails after they’ve already been delivered to the inbox.

These filters examine incoming messages based on user-defined rules, content analysis, or even machine learning, depending on the software. For example, if a certain phrase or sender consistently appears in unwanted emails, the filter can be trained to flag or redirect similar messages in the future. Many tools also include user input features, allowing manual tagging of spam or legitimate messages to fine-tune accuracy over time.

One of the main benefits of desktop spam filters is the level of control they offer. Users can customize filtering rules and manually train the software based on their personal email habits. This enables them to better manage variations of attacks, such as email salting, where attackers slightly modify emails to bypass basic filters and track responses. However, because filtering happens after delivery, these tools don’t prevent spam from reaching your system. They only help manage it once it arrives. Therefore, it’s recommended to use such filters in combination with other types of protection.

ISP spam filters

ISP spam filters are protection systems provided by Internet Service Providers (ISPs) or email service providers. They automatically screen emails without the user needing to configure anything.

The advantage of such filters is their convenience. Users don’t need to install or manage any software themselves. 

Since providers like Google and Microsoft manage billions of emails daily, they have access to massive data sets and real-time threat updates that allow their filters to adapt quickly to new spam techniques. However, users have limited control over how these filters operate, and there’s always a risk of legitimate messages being misclassified.

Benefits of Using a Spam Filter

Spam may seem like a simple annoyance, but behind many of those messages are real phishing scams trying to steal your login credentials, malware hiding in attachments, and distractions that clog up your day.

That’s exactly why using a spam filter is a must nowadays. They help protect you and your organization by filtering out threats before they reach the inbox. The key benefits include:

Common Features to Look for in a Spam Filter

Different types of spam filters have their own specific set of features. Still, there are some core capabilities that make a filter more effective and easier to manage. So, one of the most useful email protection tips is to look for spam filters that offer:

The Bottom Line

Spam filters are among the most important tools for protecting your inbox from threats that can compromise your safety and waste your time. They offer a first layer of defense against phishing attempts, malware, and message overload.

However, spam emails are just one form of email-based threat that attackers use to infiltrate systems and steal data. That is why PowerDMARC offers spam filtering as part of a broader suite of advanced email security solutions designed to keep your organization secure and compliant. Book a demo today, and we’ll help you strengthen your defenses and stay one step ahead of evolving threats.

Frequently Asked Questions (FAQs)

Can I create my own spam filter rules?

Yes. Most email platforms let you set custom rules to filter emails by content, sender, or recurring patterns.

Why do legit emails sometimes go to spam?

Spam filters can mistake good emails for spam, either due to certain words in the content, sender reputation, or missing authentication.

What should I do if my emails are marked as spam?

Check your content and settings, set up SPF, DKIM, and DMARC, and ask users to mark your emails as “not spam.”

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