What Is a Spam Filter and How Does It Work?

Avatar for Miguel Nogueira Miguel Nogueira
Avatar for Miguel Nogueira Miguel Nogueira

March 9, 2022

8 min read

What Is a Spam Filter and How Does It Work?

Firstly, let’s define exactly what a spam filter is. It’s a complex tool, but it can simply be described as a program that detects messages which may contain harmful viruses or unsolicited messages, promptly preventing them from reaching the main inbox, and acting as a gatekeeper of sorts. It is essentially, inbox protection.

Naturally, this paragraph is really an oversimplification of what spam filters really are.

In more technical terms, they are software programs that through a series of thousands of algorithmic pre-defined rules, often using heuristic methods, determine and apply a score to each email which measures the probability of them being considered spam. This is based on criteria that often update the more the algorithm develops itself and learns to distinguish ever increasingly complex patterns and trends. There’s a low point, a threshold, which triggers the algorithm into flagging an incoming email as spam and thus redirecting it to the appropriate folder.

Above you can see the first recorded case of a spam filter being used. As Gandalf the Grey (Spam filter) prevents a Balrog of Morgoth (Spam Email) from crossing the bridge of Khazad-dûm and wreaking havoc on the lands of Middle-Earth during the Third Era. (reaching your inbox).

Why should I care about spam filters?

You should take notice of spam filters for two different reasons.

  1. From a business perspective - how it affects your operations?
  2. From an individual perspective - how it affects you when you receive spam?

Speaking from the business side of things, spam is a great threat looming over the horizon.

Whether it is B2C or B2B, marketers shiver at the thought of ending up in the spam folder. It’s harmful for a handful of reasons; it harms your reputation and it directly impacts your Return on Investment. It can essentially derail your email marketing campaign.

The topic of today is relevant knowledge for your business precisely because by understanding how spam filtering works you will be much more capable of keeping up with email marketing best practices and ensuring success for your email marketing adventures throughout the wonderful world of digital marketing.

In fact, I’d say that marketers stand to gain the most from understanding how spam filters work.

However, analyzing this topic from an individual standpoint is also very important.

Spam is… well… a nuisance.

“Oh boy, more spam to read” - No one, ever

You know spam emails are forwarded directly to the spam inbox, but you too sometimes receive emails you consider spam. Yes, you mark them as such, or you delete them. Sometimes you get bored of a newsletter and unsubscribe only to find out you need to unsubscribe from the secondary newsletter as well.

This is why spam filters are an important topic. There are so many types of spam filters that even an individual email user can take advantage of to have a more stress-free experience with their email service.

After all, when you think about it, how much do you use email? I’m just guessing here, but if we look at the number of emails sent daily around the world, I’d guess a lot.

For both businesses and individuals, most spam received is relatively harmless and we have already spoken of the importance of not sending spam. However, some very few clever spammers will find ways into your inbox and it is in these cases in knowing just a bit more about filtering options and tools might give you the very edge you need in order to have a perfectly clean inbox.

In some cases, spam can actually be quite harmful, some emails can contain highly dangerous malware that could put your data’s security at risk.

While this is of particular importance for organizations in order to properly safeguard their assets and digital infrastructure, even individuals who use emails for particular purposes can be victims of these attacks. The importance of digital safety and awareness of security tools cannot be overstated.

I’m convinced! Tell me more about these spam filters

We joked about the first spam filter being used for the first time in Tolkien’s fantasy world. However, to provide real historical context, the first spam filters actually rose to existence out of desperate need, as spam in the late 90s and early 2000s was becoming a cause for serious concern. At this time, unfortunately, spam filters weren’t very accurate.

Initially, spam filters started out very simple, and would look for certain keywords or specific sentences to determine whether or not an email was spam or genuinely a Nigerian prince trying to offer you a million dollars in exchange for your social security details.

In the modern world, each email domain has its own spam filter built on machine learning technology, with complicated algorithms to determine what is considered spam or not.

Nowadays, with email being one of the most used platforms for communication between people and businesses, the need to have accurate, precise, and reliable spam filters is much bigger than before, due to the sheer volume of users exchanging emails.

Now more than ever, there is an unquestionable necessity for cybersecurity, as businesses are more and more fully digital, the need to safeguard your information and digital infrastructure is unprecedented.

It is also worth considering the growing legal requirements in order to safeguard all users of the internet as it continues to grow and be a catalyst for globalized markets.

The legal requirements are especially important when it comes to spam, as various law enactments, such as the CAN-SPAM Act of 2003 and the GDPR have very specific rules and a zero-tolerance policy for any form of spam. Not being compliant with these laws can bring several problems to your company and even to you, individually.

There are several types of spam filters, we’ll be addressing a few of them in this article, so you can become the spam filter human encyclopedia you have always dreamed of becoming.

Types of spam filters

  • Bayesian Filter
  • Blocklist Filter
  • Content Filter
  • Gateway Filter
  • Header Filter
  • Hosted-spam Filter
  • Language Filter
  • Permissions Filter
  • Rule-based Filter

Let’s get to it!

Types of spam filters

Bayesian Filter

Bayesian filters are pretty awesome, they’re some of the smartest filters around.

A Bayesian spam filter basically learns what spam is specifically to you. This type of filter learns to classify an email as spam based on your preferences, that is to say, whenever you mark an email as spam the filter will take note of the structure, language, content, and overall characteristics of the email you said was bad and learn to do so automatically in the future.

How you filtering?

Blocklist Filter

Whenever a sender of spam is detected they are usually put on a blocklist, in order to have them identified for all the email community to know.

A blocklist spam filter is a connection between your spam folder and these lists. Basically, this filter will block any emails from a sender address that is present on a black list.

Although this filter requires previously acquired knowledge to work (the blocklists) it is still a very efficient spam filtering method, as it prevents the most obvious spam emails from reaching your inbox and distracting you from getting your work done.

Content Filter

A content spam filter will take a good look at the body of the email you received and try to determine whether there is anything that could be seen as suspicious, be it certain phrases or even pictures.

As a side note and a small bit of advice, this is why structuring your content according to best practices is essential if you want to avoid the spam folder. Things such as poor image-to-text ratio or a lack of information or options that give credibility to your email (business address, unsubscribe option, etc) can be very harmful to your campaigns because the content spam filter might mistakenly consider your email to be spam.

Gateway Filter

I said earlier that, generically speaking, a spam filter is ultimately a gatekeeper that prevents bad emails from reaching your inbox. A gateway spam filter is usually a physical server, set up in order to receive the emails first, and only after approval by the filter configurations and criteria allow emails to go through to the recipient.

Header Filter

A header spam filter, as expected, checks the header of the email in order to decide whether it comes from a legitimate source or not. It’s a simple but effective way to filter incoming emails.

Hosted-spam Filter

Hosted spam filters work very similarly to gateway spam filters. The big difference is that hosted spam filters are in the Cloud and not in a physical server.

While the gateway filter works wonders in protecting your inbox by filtering emails before they even land on the server, hosted spam filters are very useful to filter whatever the gateway wasn’t ready or able to. This is possible because a cloud-based filter is much easier to upgrade due to their nonphysical nature.

Uniting the security of a physical server and a cloud-based service can be a mighty combo indeed.

Stick those filters together. Get it? Stick.

Language Filter

Email is a global powerhouse of communication.

Language filters help determine whether a bad email is coming in or not by looking at the language differences, patterns, and phrases and determining which ones are likely to be from a foreign customer or partner and which ones are just spam crossing borders.

Depending on your level of internationalization you might need to adjust and customize your language spam filter a little, in order to lower the number of false positives your filter could identify.

Nevertheless, it is a powerful tool to help keep your inbox clean and spam free.

Permissions Filter

Permission spam filters do a very good job at a very simple task.

They check if the receiving party has provided permission to the sender party to email them.

And that’s it!

Rule-based Filter

Rule-based spam filters are perhaps the most customizable filters available to you, by configuring specific triggers and conditions these filters will send any incoming email that matches your criteria directly to the spam folder.

This is very useful if your business is frequently attacked by spam of a very specific category and helps immensely in keeping your inbox clean and fresh as it allows you to minimize human error when dealing with bad emails.

Even in private use, a rule-based spam filter can be very useful if you specifically want to avoid receiving any type of messages of a certain category.

Your filter, your choice.


Let’s take a moment to revise what we just talked about.

There are many types of spam filters to help you keep your inbox clean. They often complement and synergize with each other, so it is important to take a look at the right combination of filters for you, as both an individual and as a business.

Here’s a quick overview of the filters we talked about:

Bayesian Filter

  • Bayesian Filter - Smart, self-learning filter. Evolves with time into a powerful tool.
  • Blocklist Filter - Checks lists of bad senders.
  • Content Filter - Checks the body of the email, blocks bad content
  • Gateway Filter - Physical security for your servers.
  • Header Filter - IP not credible? Spam!
  • Hosted-spam Filter - Cloud-based tool, quick upgrades, powerful combo with the gateway.
  • Language Filter - Diminishes risks with internationalization.
  • Permissions Filter - Checks permissions and blocks anyone without the secret password
  • Rule-based Filter - Game, set, match!

We hope you found this article helpful in learning more about spam filters, why they matter, and how they work.

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