Email Filtering Explained: What Is It and How Does It Work

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In a world where email communication is integral to business operations, the threat of spam, phishing attempts, and other malicious content looms large, making email filtering a critical defense strategy. This article delves into the concept of email filtering, elucidating its role and importance in ensuring the integrity and security of digital communications in today's increasingly cyber threat-prone landscape.

From businesses to individuals, email is an indispensable tool. Yet, amidst the constant barrage of emails that inundate our inboxes, it's easy to lose sight of what truly matters. 

According to a spam and phishing report by Kaspersky, 48.63% of all emails sent in 2022 were spam. And with billions of email messages sent daily, this figure is a concern. 

Unfortunately, pesky emails are not the only thing to worry about. Email is also one of the biggest tools that cybercriminals use and many companies have faced severe email attacks. Consequently,  you must also worry about malicious emails, which can compromise your network, devices, or personal information.

Email filtering provides a powerful solution — the ability to sift through a deluge of information and identify what is truly important. For an organization, employees can focus on what is meaningful and essential, while discarding distractions and noise.

Let's explore email filtering  – including how it works, benefits, and more. 

What is email filtering?

Email filtering is a process used to sort emails and identify unwanted messages such as spam, malware, and phishing attempts. The goal is to ensure that they don't reach the recipient's primary inbox.  It is an essential security measure that helps protect users from unwanted or malicious messages.

Many assume that email filtering is just for inbound emails –  this is not correct. It can also be outbound. For outbound filtering, the outbound filter features can detect spam-like behavior on an individual basis, thus preventing spam from leaving the email server's IP address. This helps protect the mail server from being blocked and ensures valid emails are always sent.

How does email filtering work? 

Email filters inspect emails for identifiable spam characteristics. After inspection, the filter will then move the emails to different folders, depending on the outcome of the analysis. For example, the filter will place a vital work email in the «Work» folder. On the other hand, it will move suspicious emails to the «Spam» or «Junk» folder.

These are the common characteristics or red flags that filters use to identify email that should not find its way to the primary inbox:

  • Known malicious content or URLs: Email filters check for known patterns or characteristics of malicious content, such as viruses, malware, or phishing links, to block or quarantine emails containing these threats.
  • Large attachments or unusual file types: Email filters may flag emails with large attachments or uncommon file types that may be indicative of malware or other harmful content.
  • Suspicious keywords or phrases: Certain keywords and  phrases are known to be  commonly used in spam or phishing emails. Examples include  «urgent action required» or «click here to claim your prize.» 
  • Misspellings or variations: Attackers may use slight variations or misspellings of well-known brands or companies to trick recipients into opening malicious emails. Email filters can identify and block these messages.
  • Unknown or suspicious senders: Email filters may flag or block messages from senders who are not recognized or whose email addresses have been reported as suspicious.
  • Forged or suspicious sender addresses: Attackers may forge the sender address to make it appear as if the message is from a legitimate source.
  • Excessive capitalization or punctuation: Spammers may use excessive capitalization or punctuation in email subject lines to attract attention. Email filters can flag these messages as potential spam.
  • Explicit or inappropriate content: Email filters can be set up to flag or block messages containing explicit or inappropriate content that may violate an organization's policies or legal requirements.
  • Malicious IP address: If an email originates from an IP address with a negative reputation, the filter could mark it as spam to protect the receiver from potential danger.

Also Read: Top Cyber Attack Vectors

Why do companies need email filtering?

The popularity of email communication has made them attractive to cybercriminals, spammers, and scammers, bringing about the need for email filtering solutions. 

For organizations, email filtering is a critical defense against ills that can cause monumental damage. A simple, innocent email can be the beginning of serious trouble for your company.

Below is a look at some of the valuable benefits that demonstrate why every organization should be using email filters. 

1. Email filters keep spam emails out of the work inbox

Spam emails are annoying and can waste your employees’ time. However, you don't have to worry about this with email filters. All flagged emails are sent to the spam folder, away from all the other important emails in the inbox. 

Third-party email filter software for enterprises can even block spam emails from reaching the inbox, which is a huge bonus.  

2. Employees are protected from email-borne threats

Spam emails are a considerable threat to cybersecurity. According to a study by cybersecurity company SlashNext, there were over 255 million phishing email attacks in 2022 (for just six months). 

Email filtering helps protect employees from these threats by scanning incoming emails for known patterns or characteristics. When the filter identifies an email that may be a threat, it can take different actions depending on the organization's security policies.

Email filters also use sender authentication techniques to verify the authenticity of the sender's email address and domain. 

3. Streamlined inbox  

According to the email marketing company Campaign Monitor, an average office employee receives over 100 business emails daily. 

This number can be staggering if you consider other types of emails, such as subscription updates, newsletters, social media updates, and spam emails. 

Without email filters, sorting through all these emails to find important ones will consume a lot of time and effort. 

4. Enhances organizational reputation 

Without proper filtering, your email servers can be infected with malware, which can be passed on to other parties when sending emails.

For instance, outbound filters can identify errant employees in your organization who can send emails that could potentially injure the company’s reputation.  Examples include employees who may be engaging in risky or malicious behavior, such as sending sensitive data to unauthorized recipients or using company email to distribute spam or malware.

5. Avoiding legal issues

Some companies can face legal issues if they fail to address certain emails promptly or appropriately. Email filtering can help companies avoid these legal issues by ensuring that important emails are identified and handled appropriately.

In the absence of efficient email filtering, it’s possible to have important emails getting lost in the massive stream of emails that keep filling the inbox.

Also Read: How to Remediate Cybersecurity Threats

Email filtering deployment: on-premise, cloud, hybrid

When planning to deploy an email filtering solution, one of the most important things you will need to think about are the options. In this case, you’ll have three to consider: on-premise, cloud, and hybrid.

On-premise is a deployment option where the email filtering software is installed and managed locally on the organization's servers. This option offers organizations more control over their email filtering solution and data, making it ideal for organizations with strict security or compliance requirements. However, on-premise email filtering can be more complex to set up and maintain, requiring specialized IT expertise and resources.

In the cloud option, the filtering service is delivered over the internet and can be accessed from anywhere, provided there is an internet connection. 

Further Reading: Cloud vs On-Premise Cost Analysis

The hybrid approach combines the benefits of both on-premise and cloud email filtering. This means the email filter is deployed both locally and in the cloud, allowing the organization to balance control and security with scalability and flexibility. This option can offer the best of both worlds, but it may require more complex configuration and management.

Common types of spam email 

The actors are constantly changing tactic. But the key goal of their emails is always to get the recipient to take some action. The desired action plus intended outcomes make it possible to group spam emails into these major categories: 

1. Financial scams

Employees can receive emails that promise huge monetary offers if they can do a favor for the sender. 

A good example is the «Nigerian Prince» scam, where a sender claiming to be a Nigerian prince asks the potential victim for help accessing their inheritance. The victim will then be asked to send money for legal fees or something else, with a promise to be repaid with a bigger reward once the ‘Prince’ receives the ‘inheritance’  money. 

2. Promotions  

According to SpamLaws, ad-related emails are one of the most common forms of spam, accounting for 36% of all spam messages. 

These messages are usually unsolicited adverts from companies trying to promote their products or services. They often come with enticing offers, such as substantial price discounts. Some adverts might be genuine, while others may be a scam.  

3. Phishing emails

Cybercriminals use phishing emails to steal sensitive personal information, such as usernames, passwords, bank accounts, credit card numbers, etc.

According to a 2021 Cisco cybersecurity threat report, phishing emails accounted for about 90% of internet data breaches. Phishing emails are dangerous as they are designed to appear legitimate by mimicking the email address of authoritative senders. 

For example, a spam email might have the URL address «» instead of «» Unfortunately, most people overlook such typos in phishing emails and end up giving out important information. 

Also Read: Clone Phishing & Prevention Tactics

4. Trojan horse emails

A Trojan horse email is a type of malicious email that contains an attachment or link that appears harmless but, when opened, unleashes malware or other harmful software onto the recipient's device. 

The email may be disguised as a legitimate message, such as an invoice or shipping notice, to trick the recipient into opening the attachment or clicking on the link. Once the Trojan is installed, it can be used by attackers to steal sensitive information, monitor activity, or control the system remotely.

5. Fake malware warnings

These emails often claim that the recipient's device or network is infected with malware and encourages them to download a software update or click on a link to fix the problem. 

However, the update or link actually leads to a malicious website or file that can infect the user's device with malware.

Types of email filtering systems

Which email filtering system can you use to filter spam emails for your organization? Here are the most common types:

1. Filters provided by email service providers

Email service provider filters are built-in filtering systems offered by most email clients. Therefore, it’s possible that your employees are already using one, at least at a personal level. These include services such as Gmail, Hotmail, Outlook Mail, Yahoo Mail, and others. 

The built-in filters use advanced techniques to scan emails as they enter the users’ inboxes. They then organize the messages into different categories. For example, Gmail filters will organize emails into «Primary,» «Updates,» Social," «Promotions,» etc. Flagged emails are also sent into «Spam» or «Junk' folders. You can also set up custom filters to organize emails into more categories, such as „Work,“ „Personal,“ etc. 

2. Email filters designed for desktop use

Desktop email filters are software or programs that are downloaded and installed on desktop computers. They include free or premium programs, which can scan messages entering inboxes and flag any suspicious emails.

In addition, some computer security systems also work as desktop spam filters. For example, most business antivirus programs can monitor emails for viruses and malware. 

3. Email filters provided by third-party vendors 

Third-party email filters are often cloud-based spam filtering software. They leverage advanced and proprietary filtering techniques to offer protection from email-borne cyber threats and unsolicited messages.

This category of email filters is the most ideal for enterprises, as they are more effective than desktop or email service provider filters.  

Email filtering techniques

Email filtering solutions use several methods to flag spam and suspicious emails. Below is a look at the top ones: 

1. Content filtering 

As the name suggests, content filtering uses content analysis to monitor and flag suspicious emails in the inbox or outbox. The email filtering system will analyze the text, flagging down trigger words such as „Free,“ „You have won,“ etc.

Content-based filtering can also monitor email headers and messages, flagging suspicious features. The content filters will also analyze and flag suspicious links and attachments. 

2. Blocklist filtering 

Also known as blacklisting, this technique involves blocking emails from specific senders or IP addresses. The email filtering system generates the block list based on the sender's reputation score. 

For example, the filter will likely block a sender flagged by multiple recipients. Therefore, if they try to send an email, the built-in filters will flag it down and move it to the spam folder. 

3. Safelist filter 

Safelist filtering, or whitelisting, works in the opposite manner of blacklisting. The email filters only allow messages from a pre-approved list of senders. Other emails are flagged and moved into a quarantine folder. 

4. Temporary block list filtering

Also known as greylisting, this technique works similarly to blacklisting and whitelisting. The email filters temporarily reject incoming messages from unknown senders with a resend request. 

If the sender is genuine, they will resend the email again, confirming its legitimacy. However, if the email is from a spammer, they will likely ignore the resend request. 

5. Antivirus filtering 

Antivirus filtering is a common technique for email spam detection. The email filtering system scans incoming and outgoing messages for viruses and malicious programs. Any flagged emails are then blocked or quarantined. 

6. Rule-based filtering 

Rule-based filtering is a custom email filtering technique where the network administrator creates filtering rules. For example, they can set custom rules in users’ inboxes, where emails from a specific sender or subjects are moved to the spam folder. 

Alternatively, they can customize the built-in email filters in such a way that they automatically never flag specific emails as spam and instead send them to the „Primary“ folder. 

6. Adaptive email filtering 

In this technique, the email filtering software learns and adapts its filters to increase efficiency.

It does this through machine learning & artificial intelligence, Bayesian modeling (predicting probability), email users’ feedback, etc. 

A good example isGmail’s AI-powered spam filters, which flag down over 99% of spam emails.  

7. Language filtering

In this technique, emails are sorted based on the language used in the message. It can be used to filter out unwanted emails that are not written in the recipient's preferred language. The assumption is that the user has a default language (s) that all emails come in. Therefore, any email that comes in an unfamiliar language is suspect. 

For example, if a recipient has set their email filter to only accept messages in English, any incoming email in a different language will be filtered out and sent to the spam folder.

Language-based email filtering can also result in false positives, as not all emails written in a foreign language or containing certain keywords are necessarily spam or malicious.

8. Header filters 

The email message headers are scanned for specific information that can help identify and block unwanted emails. The headers of an email message contain metadata about the message, such as the sender's email address, the date and time the message was sent, the email subject line, and the email client used to send the message.

Header filtering typically uses a set of rules or algorithms to analyze the header information and determine whether an email is legitimate or unwanted. For example, a header filter may examine the sender's email address to see if it matches a list of known spammers.

One advantage of header filtering is that it can be a relatively lightweight and efficient method, as it does not require scanning the entire email message. This can help to reduce the processing load on email servers and improve the overall performance of the email system.

9. Challenge-response filters

The email filtering system sends an automated challenge message back to the sender to verify whether the message is legitimate. The sender must then respond to the challenge before the email is delivered to the recipient's inbox.

The challenge message typically contains a code or a link that the sender must click on to verify their identity or intent.

Choosing an email filtering service: what should you consider? 

The effectiveness of an email filter will directly impact the security and productivity of your email system. 

It’s a critical decision that should be guided by these factors:

  • Reliability and uptime: Do they have a track record of reliability and uptime? Can they provide  a service level agreement (SLA) that guarantees a certain level of availability and performance.
  • Technical support: Go for a provider that offers 24/7 support, and has knowledgeable and responsive support staff that can help resolve issues fast.
  • Integration: Make sure that the service is compatible with your existing email system and can be easily integrated with your other security and productivity tools.
  • Cost: Evaluate the cost of the solution, taking into account any setup or installation fees, ongoing subscription fees, and any additional costs for customization, technical support, or other services. If you are not familiar with IT cost reduction strategies, please check this guide for comprehensive insights.
  • Reputation: Read reviews and testimonials from other customers to get a sense of the provider's strengths and weaknesses.

Corporate vs free email filters

Corporate email filters are more robust and customizable than free email filters. They can also be tailored to the specific needs of your organization. Corporate filters also typically come with a higher level of technical support and maintenance, which can be important for organizations that rely on email as a critical communication tool.

However, corporate email filters can also be expensive, and may require additional hardware or software to be installed and maintained. This can add to the cost and complexity of your email system.

Free email filters, on the other hand, are typically available as a standalone tool that can be easily integrated with your existing email system. They are often easier to set up and use than corporate email filters, and can be a cost-effective option for small businesses or organizations with limited budgets.

However, free email filters may not offer the same level of customization or technical support as corporate filters, and may not be as effective at detecting and blocking advanced forms of suspect emails.

If yours is a larger organization with complex email requirements, a corporate email filter may be the better option. If you are a smaller organization with more limited resources, a free email filter may be a more practical choice. 

How to check if your company’s emails are going to spam

While we have focused more on incoming emails, it’s also critical to understand that your own company’s outgoing emails can also go to spam or be blocked altogether.

How do you confirm this and avoid it?

  • Check your organization’s email reputation: There are several online tools available that allow you to check your email reputation, such as Sender Score, which can help you determine whether your company's email domain or IP address has been flagged for spam.
  • Authenticate your emails: Implement authentication protocols such as Sender Policy Framework (SPF),  DomainKeys Identified Mail (DKIM) and Domain-based Message Authentication Reporting and Conformance  (DMARC)
  • Monitor your email delivery: Use an email delivery monitoring service to track the delivery and engagement of your outgoing emails, including whether they are being delivered to the inbox or spam folder.

Other ways to prevent spam emails 

Yes, email filters are excellent at flagging spam and malicious emails. However, some messages will still slip through the cracks and can cause damage if malicious. So, which other ways can help protect against spam?

1. Hardware and software upgrades

Using devices with the latest hardware and software can offer more protection from spam emails. For example, the latest phones and computers support the latest operating systems and device software, which often have built-in anti-spam protection. 

You should also keep all software up-to-date, especially antivirus and browsers, giving you an extra layer of protection from harmful spam.

Also read: What is a Legacy System and Software?

2. Encourage employees to exercise discretion 

Some email addresses in your company may be in a spam mailing list depending on how and where they are used. For example, if some employees regularly use their work email to make purchases, such information can be shared with marketers.

Consequently, they will start seeing an upsurge in advertisement emails with unsolicited offers. Additionally, the company’s email addresses can be grabbed from public platforms such as social media accounts and shared with marketers and other spammers. 

Train your employees to be careful about where they use their work email addresses. Mandate them not to use their work emails, for example, to purchase personal items or on social media. This way, their work emails will only be for official tasks. 

3. Inculcate a culture of detecting email-based scams 

Teach the entire staff  to act as the final filtering system. Emphasize on the need to carefully review each email and look for common signs of spam messages, such as: 

  • Inconsistent email addresses, especially ones with typos 
  • Grammatical and spelling errors
  • Suspicious tone, especially greetings
  • Unusual requests, often with threats or a sense of urgency 
  • Unsolicited emails, especially ones claiming that the receiver has won money or a prize
  • Emails with suspicious links and attachments. 

Also Read: Deepfake Threats

Final remarks 

Spam emails are growing at an alarming rate. Of course some are harmless, but others are not — criminals can use them to carry out devastating cyberattacks. This is why email filtering has become a vital first line of defense against these attacks. 

Besides cybersecurity purposes, email filters also keep the inbox organized by placing each email in its appropriate folder. This can save the time that would otherwise have been spent sorting through hundreds of emails.  

While at it, please make sure that your employee training also extends to their personal email addresses. Although it may appear that employees’ personal emails don't affect business security, they can become a liability if they're used to transfer sensitive information or discuss company-related matters. Educate your teams on how to use email filters for their personal accounts as well.

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