Why Spam Filters Hate You

Vladislav Podolyako
May 27, 2021
Reading duration

Provocative title aside: No. Spam filters don’t hate you, albeit you may get such an impression after several emails get redirected to spam folders. However, the world of cold email outreach isn’t managed by emotions. Algorithms and machine learning drive it. Machines cannot hate, they can only observe, connect the dots and take measures based on users’ feedback. 

Befriending spam filters is your top priority if you want your emails delivered and wish your sender reputation grew faster. But to many spam-sending newbies, spam filters are a mystery to be explored. How do they decide which emails are good to go and which ones should be penalized? How to avoid spam filters during your email marketing campaigns? What kinds of spam are they looking for? 

Fear not: With this blog post, the concept of spam filters will become a lot more transparent and less intimidating. 

Together, we will explore the following:

What is spam filtering?

Spam filtering is the process of detecting any message that is unwanted, unrequired, and filled with dangerous malware. Such a message gets marked as spam and is denied access to the recipients’ inboxes within this process.

Spam filtering is used by:

  • Internet service providers (ISPs) — they must ensure that their users' privacy isn’t compromised and no personal or business data is leaked to third parties. 
  • Business enterprises they are used to ensure the safety of financial transactions and protect business email correspondence from data thieves and cybercriminals. 
  • Small companies and startups they are particularly vulnerable to spam attacks and, therefore, must build a strong sender reputation and fortify their network. 

Spam filtering concerns both emails that arrive in inboxes and emails sent from the company’s network. Due to this, there is a large variety of spam filters and spam filtering types, from the cloud-hosted ones to various built-in solutions.   

How do spam filters work?

Any spam filter is basically a set of algorithms. Each of these algorithms evaluates every incoming email component and outlines its potential spamminess via a numerical score. If this score is high enough, the message is labeled as spam and goes to a designed spam folder instead of the recipient’s inbox.  

As emails became more complicated with time, spam filters grew more scrupulous in their assessment. Within each spam filtering system, a set of filters is dedicated to analyzing each component of an incoming message. Additionally, spam filters evaluate user behavior and collect user reports to stay aware of new spam trigger words and problematic senders.  

Main types of spam filters 

There are many gateways that spammers can use to worm their way into business correspondence and affect vulnerable data. To prevent it, each gateway is guarded by an individual spam filter, designed specifically for apprehending the incoming spam and making sure it will never be seen or opened by an unsuspecting recipient.  

Let’s introduce these vigilant defenders, one by one. 

Content filter

This filter scans the body text of incoming emails, looking for spam trigger words and assessing whether the email’s content can be considered spam. For example, if it keeps finding the word “buy” over and over, it will give a high spam score and block it from reaching the recipient’s inbox. 

Content spam filters are the simplest, most popular solutions for spam filtering. They’re good for intercepting common types of spam, but the obvious downside is that they can block harmless mail if it contains spam trigger words. It’s worth noting, however, that the latter occurs if legitimate senders don’t pay attention to their Sender Score and don’t take enough measures to improve domain reputation. 

Header filter 

While content spam filters explore what is in the email, header spam filters pay close attention to who sent the email by inspecting the email’s header. Mind that the email’s subject line is not a header. 

For example, this is what a Gmail header looks like. 

Email headers display valuable information, such as: 

  • Message ID. It’s a message identifier generated by the sender’s email service specifically for this particular message. There can be no two identical message IDs, so this element is used for detecting forged email headers. 
  • Sender address. Header spam filters check where the incoming mail comes from and consult blacklists to check the sender’s domain reputation. If they find out that the sender’s domain has been marked as suspicious by an Internet service provider, they will instantly label the message as spam. 
  • DNS records. Email’s header allows you to take a look at DNS records of the sender, making it possible to check the sender’s SPF, DKIM, and DMARC policy. 

You can view the header by opening any of your incoming emails, clicking More in the upper right corner and selecting Show original. Additionally, you have Download original and Copy to clipboard options for preserving this data and presenting it to your tech support in case it’s necessary. 

Language filters

As email correspondence grew more international, language filters became a thing. Some spammers often target users outside their geographical area — after all, they don’t want their recipients to read their message. They want them to open an email, click on a link or download a file. Knowing the recipient’s language isn’t necessary for provoking them to act. 

Language filters exit to ensure that you receive emails that speak your language by scanning mail and removing foreign emails. The use of these filters is quite limited and you get to configure them yourself in order to strengthen security. 

Rule-based filters

The most flexible and customizable anti-spam solutions, rule-based filters (also known as user-defined filters) are your best friends if you run a business. Since many cybercriminals are eager to get their hands on your customers’ data, your employees’ data, your financial information, you must take extra measures to protect your inbox from unsolicited emails and malware. Rule-based filters allow you to build a system of rules that defines which email is good to go and which should be blocked on sight.

For example, you have the power to greenlight and block: 

  • Senders 
  • Words 
  • Domains

Within a rule-based spam filtering system, a message is given a zero score. After this, your filters parse it and evaluate it, ultimately giving it a score. If this score exceeds a certain threshold, the filters treat this email as spam. 

As customizable as they are, rule-based filters offer a large variety of configuration options, which may lead to slow performance. Additionally, spam senders can bypass some of the rules, for example, writing customized forbidden words in a certain way (“B*U*Y” instead of “buy”). 

Blacklist filters

Just as popular as rules-based filters, blacklist filters scan through lists of domains and IPs that have been known for breaking email outreach guidelines. If they find the sender's address in one of the blacklists, they instantly block access to the inbox. 

Whitelist filter

Whitelist filters base their decisions on the recipients’ whitelists, i.e., addresses and IPs that are allowed to send mail. This is why such filters are also called permission-based filters. 

Greylist filter

An in-between option, greylist filters base their approach on sender behavior. Whenever an unsolicited email arrives in an inbox, it’s instantly rejected, and the sender receives a corresponding notification with a request to re-send their message five minutes later. However, if the sender tries again, this email will be accepted. This principle implies that a legitimate sender will follow the request and send the message again, meanwhile, a spammer won’t be bothered with such instructions. 

Bayesian filter 

A Bayesian spam filter explores your user behavior to offer a flexible and intelligent defense from spam. Whenever you send an email to a spam folder, a Bayesian filter memorizes the components and elements of the email you considered suspicious and adds these features to the database. Due to its ability to learn and adapt to new threats, a Bayesian spam filter provides more sophisticated ways of keeping your inbox safe. 

As effective as it is, any Bayesian filter, however, needs more time than other filters before it can protect your outreach at its full capacity. You can help it by being an active user, diligently organizing your inbox, reporting spammy content, and notifying your mail servers about suspicious users. 

Spam filter categories

Spam filters come in 3 varieties. Let’s explore them by taking a look at a set of different solutions: 

If you use Gmail or Outlook, you already use their spam filters. However, for extra protection, you can go for additional solutions that provide increased security for your business correspondence and let you protect your employees. 

Ways to improve email deliverability

So, with such an abundance of spam filters, you may ask yourself, “How do I avoid being marked as spammer? There are so many solutions to keep in mind!” 

Fear not.  It’s not that scary as it sounds. 

As you might have noticed, the majority of spam filters explore your reputation as a sender before their verdict is in. Also, different types of spam filters check your outbound message simultaneously. Therefore, even when content filters give your body text a less than satisfactory score, it won’t be an issue if your IP address and domain reputation are good to go. Some great news already, right?

Once you know how spam filters work and what catches their attention, you know how to get in their good graces. 

  • Watch out for spam trigger words.  Don’t take your chances with smart content spam filters. It’s important to take a look at the latest spam trigger word lists and see if your body text has those. In case you have them, work on editing those words out of your body text or replacing them with less risky ones.  
  • Organize your mailbox. If you want to get your outbound marketing in order, start with yourself. For example, the default user-defined filtering system for Gmail breaks your inbox down into several categories. 

  • Primary.  Your main platform for business correspondence. This is where you get your sales introduction emails and communicate with prospects.  
  • Social.  This is the folder for your social media notification emails. If you receive a new InMail on LinkedIn or an invite from a user, all the messages will be directed to this section. 
  • Promotions. This is where you receive notifications from all services you’re registered in or subscribed to. This section contains newsletters, new offers and information on new policies. 
  • Updates. This folder contains all mentions of you in GoogleDocs, Trello, Jira and other services you’re registered in. Basically, it’s the folder for your personal Bat Signals. 
  • Forums. It’s an important folder for your network. This is where you can view emails from your CEO or CMO, get team updates and generally stay connected to the processes within your company. 

Much like rules-based filters, user-defined filters can be customized beyond their default settings. You can add new folders based on your needs. Emails that fail to meet any of your criteria are sent to Spam

  • Keep your DNS records in check. A healthy sending policy means healthy outreach. In modern email marketing, all senders agree to certain behavior guidelines. For example, they’re obliged to prove that they are legitimate senders and instruct recipients on what to do with shady users that seem to send messages on their behalf. You can participate in this process by providing an SPF, a DKIM signature and a proper DMARC policy. 
  • Have a proper sender ID.  Visibility and transparency are the pillars of gaining spam filters’ trust.  The more information you provide about yourself, the easier it will be for you to build a genuine relationship with spam filters. Therefore, make sure that your sender ID has a photo, a credible sender address and information about your company. 
  • Run deliverability tests. Spam filters grow and change all the time. Sometimes, the techniques that used to work are no longer enough — for example, this is why every major Google algorithm change messes your SEO and outreach up. But if you stay alert and keep looking for areas for improvement, you’re guaranteed to come out on top. The best way to do it is to use email audit tools. For example, Folrderly deliverability tests instantly show you the state of your DNS records, your relationship with Barracuda and Spam Assassin, and even scan your blacklists. It’s a simple and easy feature that saves you from a headache. 

We hope this post has debunked some of the scary myths regarding spam filters. Everything can be resolved, as long as you explore the issue thoroughly, use the right tools and follow the rules. Don’t hesitate to try our deliverability test to see what your relationship with spam filters really looks like. 

Vladislav Podolyako
Vladislav Podolyako
Founder & CEO
Vlad’s decades of entrepreneurial wisdom and business building experience have allowed him to successfully mentor a diverse group of business owners, entrepreneurs in growing their companies. A recognized expert in the areas of transforming organizational culture and leadership development, B2B Sales, Marketing, spent more than 10 years building technology products, with a background in communication networks and electronic device engineering.

Also you may like

How Does a Spam Test Work?
How Does a Spam Test Work?

For many inexperienced senders, “spam” is something that comes to their inboxes. However, in the world of B2B email marketing, “spam test” means much more — and if you want to succeed, you have to research this term properly.