8 Email Spam Filter Types (Plus a Better Way to Block Spam)

To most people, email spam is a harmless nuisance. It’s a poorly-worded offer for hair loss treatment or a free cruise that ends up in your Junk Mail or Promotions folder, thanks to a built-in email spam filter.

But when it comes to your business, spam is more than an inconvenience. In many cases, spam emails represent a vulnerability that spoofers or scammers can exploit. And cybercriminals send more than a trillion spam emails each year.

That’s because volume works: The FBI’s most recent report shows $6.9 billion in losses for 2021. So, businesses should expect these malicious attempts to only become more common.

With that in mind, email spam blockers are crucial to organizational security. Thankfully, various email spam filters can keep you, your employees, and your organization safe from mass email-based malware and phishing attempts.

What Do Email Spam Filters Do?

Email spam filters analyze email messages to determine if they’re relevant. Then, through various techniques, these blockers automatically scan incoming emails and place them into one of three categories:

• Spam – If your spam filter identifies an email as unwanted or inappropriate, it will move the message to a dedicated spam folder. In some cases, the filter will auto-delete the email instead.
• Not spam – If your spam filter recognizes an email as legitimate, it will allow the message to reach your inbox untouched.
• Potential spam – If your spam filter can’t determine the validity of an email, it may place the message in a dedicated quarantine folder. From there, you can mark the email as spam, and the filter will (usually) remember this decision for future messages.

Through this email analysis, spam filters try to keep scams, advertisements, and other undesirable content from reaching your inbox.

How Do Email Spam Filters Work?

While the precise answer to this question depends on the type of filter you employ, most email spam blockers use a Simple Mail Transfer Protocol (SMTP) server to screen for telltale signs of spam. These include:

• Unsolicited attachments – Attachments (especially from unknown senders) can contain malware. Spam filters quickly move emails with malicious attachments to a quarantine folder.
• Known IP addresses or domains – Some filters check each message against an established list of domains and IP addresses with a reputation for sending spam.
• Unknown senders – On the other hand, an unfamiliar or anonymous email address can also trigger a spam filter.
• Specific language – Poor grammar, generic introductions, and phrases that convey a sense of urgency can alert a spam blocker. 
• Bulk emails – If your spam filter can tell an email address has sent numerous bulk emails, it may flag all messages from that account.

While most email services have built-in spam filters that leverage some or all of these tactics, they often need more sophistication to catch more advanced spam. As such, it’s worth combining several types of email filters to achieve maximum protection.

8 Types of Email Filters

Since the meteoric rise of email spam in the mid-1990s, various techniques have been developed to combat unwanted email messages. The following types of filters are some of the most widely used solutions today. Spam filtering products will typically combine some or all of these filters to provide as much coverage as possible.

Content Filters

Also known as a keyword filter, a content filter scans an email’s text for typical signs of spam. Content filters commonly look for:

• Generic greetings – Emails that include your name are more likely to be legitimate; messages that begin with a greeting like “Dear Customer” are often cause for concern.
• Requests for personal information – Senders that ask for your social security number or bank information are rightfully treated as suspicious.
• Unusual spelling or grammar mistakes – Savvy scammers sometimes write unpredictably to bypass filters.
• Time-sensitive language – Phrases like “Limited time only!” or “Act fast!” are often indicators of a scam.

Content filters often have a built-in list of words and phrases to flag, but you can also add to the list as needed.

Header Filters

Header filters analyze each email’s header. In HTML emails, the header is a code snippet that conveys details about the email’s sender (such as their IP address) and the route it took to reach your inbox.

When a header filter is active, scammers with flagged IP addresses can’t simply create a new email address to bypass a denylist.

Reputation-Based Filters

These filters pull from lists of known or likely spammers and block any correspondence tied to their email accounts, domains, or IP addresses. The lists (known as Reputation Block Lists) are often publicly available, though some spam filtering services keep proprietary records.

Anti-Virus

While not a spam filter per se, anti-virus technology is usually included in spam filtering services. An anti-virus solution helps protect email users from the dangers of unsolicited emails by analyzing code.

Specifically, anti-virus filters scan for known and zero-day malware hidden in emails using signature- and behavior-based detection tech.

Denylist Filters

Denylist filters allow your organization to choose specific senders to block. Any email received from one of these senders is automatically hidden or deleted.

You may see other terms for “denylist,” such as:

• Blocklist
• Exclude list
• Avoid list
• Blacklist (though this name is declining in popularity)

Safelist Filters

Safelist filters are the opposite of denylist filters – they allow organizations to choose which email senders can make it through the spam filter. As a result, safelists are ideal for businesses that only want to receive messages from select accounts.

Other names for “safelist” include:

• Allowlist
• Include list
• Prefer list
• Whitelist

Language Filters

Language filters ensure that you only receive emails in your preferred language. In some ways, language filters are less about eliminating spam and more about reducing inbox bloat.

While language filters are beneficial, they can be challenging to manage for businesses with an international presence.

Rule-Based Filters

A rule-based filter gives you more control over what your spam blocker accepts or denies. While content, header, and language filters are often pre-programmed to screen for particular words or phrases, rule-based blockers allow you to set the filter’s triggers. You can filter specific senders, words, or domains from one location.

Armorblox vs. Traditional Spam Filters

All of these email spam filters work together to keep your organization’s inboxes uncluttered and vulnerability-free. But even in conjunction, these eight filters can’t eliminate all unwanted content, like graymail.

Graymail is email that toes the line between spam and desired (but often excessive) emails, like marketing materials, newsletters, and recon emails that can look harmless at first sight. While competitors often miss this distinction, we don’t. Armorblox’s automatic graymail detection policy identifies and classifies these types of graymail, automatically removing emails from end-users’ inboxes that are both harmful and a nuisance.

Through the precise detection and accurate classification of graymail, Armorblox not only saves valuable time for security teams, but also reduces the amount of unwanted emails end users have to manually spend time dealing with daily. This improves not only employee productivity, but also reduces frustration for end users.

Graymail is just one of the many types of malicious emails that Armorblox flags to protect your business. Take a quick product tour today to learn more about our future-proof solution for comprehensive email security.