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How Do Email Spam Filters Work?

Email has been and will always be a central communication aspect of any business. It’s one of the core channels internally and externally.

Unfortunately, cybercriminals are also well aware of this, which is one of the reasons phishing scams have always been the preferred attack vector for bad actors.

Throw a global pandemic into the mix, a large remote workforce across the country, and a limited knowledge among people about how to counter security threats and you have a perfect environment for hackers to thrive in.

According to data collected in September 2020, the frequency of phishing threats rose considerably throughout the last few months, with companies experiencing an average of 1,185 attacks every month.

And what does this mean for organizations? It means getting yourself a filter that will shield your company from spam and prevent phishing attacks.

Let’s take a look at how they work and why not all spam filters are equal.

Define “Spam Filter”

First things first, let’s just briefly describe what spam filtering is.

Spam filters are designed to recognize unwanted, malicious emails from arriving in your inbox. Businesses typically use spam filters as a way to stop their employees receiving and falling victim to spam.

Spam filters are often used for inbound and outbound mail, but for businesses and the vast majority of people, incoming mail is what they’re most interested in vetting.

Most spam filters today are cloud-based, but there are options for on-premises solutions, too, if desired.

How Do Email Spam Filters Work?

Don’t I have a Spam Filter Already?

Most email providers, like Outlook and Gmail, will have built-in spam filters—hence the existing spam folder you likely have.

The protections offered by these, however, are limited in use and typically not enough for a business.

Spam filtering is the not the primary function of these providers, and it’s not unheard of for them to have issues.

Last July, for example, Google experienced substantial issues in effectively filtering spam, with users all around the world complaining about receiving an influx of malicious emails.

This might be acceptable for the average user, but for businesses the risks of falling victim to phishing attempts justify the need for a spam filtering service that can be relied on.

Cyberattacks cost businesses an average of $5 million—most organizations cannot afford these kinds of losses.

What Do Spam Filters Look For?

Spam filters assess incoming mail and assign it a score based on a number of various factors. Different filters use different methods of assessment, so some emails may pass the criteria for some and other fail others. This type of filtering is commonly referred to as a heuristic filter.

Here are some of things it looks for:


Where did the email come from? Is it an address that raises suspicion or is it validated? 


The reputation of some addresses precede themselves—a quality spam filter will recognize previously flagged addresses and blacklist them.


What’s in the email itself? Are there any suspicious links, attachments, or spammy language in the copy? If so, it will be filtered.


This is mostly to do with whether the recipient has opted into receiving emails from a certain address. For example, newsletters that the user has no history of receiving will be filtered more aggressively than a monthly recurring newsletter they’ve been receiving for the last year.

What Does Your Business Need?

Organizations need a good filter that will effectively reduce the amount of spam that reaches their employees.

In combination with education about what staff should look out for, this is an effective method that significantly reduces the probability that a business will be a victim of cybercrime.

How Do Email Spam Filters Work?

What Should You Expect from Your Spam Filter?

Threat detection

Modern filters will often have some form of integrated threat detection solution.

This means that it will use AI and machine learning to analyze trillions of data points in order to get a better understanding of how attackers shift their approach and what should raise a red flag.

This involves the scanning of message content and attributes, as well as domains and addresses associated with malicious intent, and other anomalies to know what to filter and what to allow.


It’s often a good idea for a business to have a solution from a business that already has a large customer base.

For example, at Impact, we use Proofpoint for our clients. Because Proofpoint has an established base of more than 100,000 customers, they have a lot of data to draw from and assess.

In other words, if there’s a new attack vector or bad actor, they likely will have already monitored and responded to it because they analyze more than two billion emails a day.

Always consider your solution provider and ask yourself if they have the data at hand to be effective in using AI and machine learning.

Employee risk

As much as you need to rely on spam filters to combat cyberattacks, it’s also important to get a handle on your biggest risk factors as far as your employees are concerned.

A quality spam filter solution should also be able to provide you with this knowledge. This can be through sending simulated phishing emails to users to determine who the employees most likely to open a phishing email are.

It also means assessing who the most targeted users are. If cyberattackers are specifically targeting someone who regularly handles sensitive information, this is information that you need to know in order to avoid becoming a victim.

This ties more broadly into education, which is vital in protecting your business from attack and creates one more hurdle for cybercriminals.

Hackers will always favor the path of least resistance—the business that has not trained its employees to spot phishing attempts stands a far better chance of being attacked than the business that has.

Training modules should be expected from a good spam filter solution.

Bottom Line

Spam filters use modern technology to prevent malicious communications reaching your inbox. While standard filters are included by most email providers, they are not by themselves as effective as a standalone solution.

Organizations should consider implementing a quality spam filter that utilizes modern technology such as machine learning and AI, as well as offers employee education, so that they can avoid becoming a victim of cybercrime.

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