How Email Spam Filters Work and Ways to Safeguard Against Them

BySuryanarayan Pal


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What is spam? Spam is nothing but unsolicited emails sent in bulk. Emails sent without strategy are considered spammy. The spam folder is one dreaded place for email marketers to be in. The good news is you can learn how to control the percentage of emails landing in spam.

Table of contents

What is a spam filter?

Spam filters for email are virtual walls that block unsolicited, malicious code containing unwanted and virus-carrying emails from reaching the user's inbox. It is a software application that protects users from spam. About 14.5 billion spam email messages are circulated daily. That is almost 45 percent of the regular email traffic in the world. Internet Service Providers (ISPs) use spam filters to ensure they do not deliver corrupt incoming emails or links to the receiver.

How do spam filters work?

Spam filters use a "heuristic" approach, which means that thousands of rules are tested on each email message before concluding them safe. Spam filters are applied to both incoming emails and outbound emails.

An email is scored based on various criteria for different types of emails using several algorithms. Each rule assigns a numerical score to measure its likelihood of being spam. If the score reaches a certain predefined amount, the email is marked as spam and blocked from heading any further.

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What is a spam detection system?

When an email is received, different types of filters look for different kinds of clues to decide its legitimacy. Using different filters can affect inbox placement when a message navigates from the sender to the subscriber's inbox. The three main spam detection systems are explained below.

1. Gateway spam filters

These are physical servers that have been positioned at the border of the organization's network. They operate as the first defense layer in preventing spam of a mailbox provider. Depending on all the incoming emails, this filter learns what to classify as spam. But on the flip-side, it has less email information to learn from than in a hosted spam filter.

2. Third-party (or hosted) spam filters

These companies have developed a standardized method of using content and credibility metrics to differentiate spam from valid emails. A third party incorporates technology to run hosted spam filters. However, the client still maintains complete control over their filtering practices.

As these spam filters have a massive customer service list, they also tend to have a broad array of data to evaluate whether to send a message to the inbox, spam, quarantine folder, or block it entirely.

3. Desktop spam filters

They are a type of third-party spam filter that exists on the end user's computer. They are customized to fit just right for the customer, and hence they are among the more difficult filters to pass through. These specialize in eliminating spam hookup emails containing 'adult' contents and can be activated for underage children's security.

Types of spam filters

Spam filters have evolved over the years and become more stringent. Given below are a few kinds of spam filters, each specializing in a specific arena.

1. Content filters

These filters check the words and content inside your email messages to determine whether they are spam or safe.

Each section and element of the email is included while screening- subject line, header, footer, photos, color, font, attachments, links, etc.

A few quick guidelines to avoid encountering the content filters are:

  • Avoid using shortened URLs,

  • Avoid overusing images and links,

  • Proofread your content for typos or grammatical errors,

  • Avoid 'spammy' or triggering words like free, donate, etc.

2. Rule-based filters

Rule-based filters work by using a set of ranking rules and filter the content based on these rules. Usually, any received message score is zero. Then the email is sent for analysis to detect if it has an already existed rule. If it does, then the weight of this rule is added to the final score. But if the Final score surpasses a specific threshold, it is filtered as spam. Otherwise, the email is valid.

A classic example of rule-based filtering is blocking emails based on the sender IP that was used to send the email. If the sender IP belongs to a blacklisted range, the email is blocked/quarantined or the email is delivered. To avoid rule-based filters, adhere to spam regulation policies and provide valuable content to your recipient.

3. Bayesian filter

This is a statistical approach for analyzing the header and content of an incoming email message to assess spam emails' likelihood. It compares the usage of terms or phrases typically associated with spam emails.

The Bayesian system works by categorizing email into groups such as "trusted" or "suspect" depending on a probability number.

To avoid the Bayesian filter, use an official email source with a good sender reputation, warm-up your domain, and have effective recipient interaction.

How can spam filters help you?

Spam filters are not present to create obstacles in your path but help you in the long run. When you follow all the rules put down by spam filters, your ISP (internet service provider) gets the hint of your trustworthiness. Not only are no or some spam reports an indication that your campaign has been well received and targeted, but it also enhances your image and could lead to improved email deliverability in the future. This will, in turn, help you reach maximum accounts and grow your business.

Do you need a spam filter?

While spam filtering might look like an option, it's more mandatory than you think. Spam is always risky.

  • A significant reason why spam filtering is so critical is that it can contain malicious material that can spread viruses and cyber-attacks towards your organization.â €

  • Most spam filters origin from static, machine learning, and dynamic analysis to detect new threats. Hence it keeps learning and evolving with every new entry of data.

  • They safeguard you against a Denial-of-Service (DoS) attack. DoS attack tries to overload your systems with an overwhelming number of requests resulting in slowing down your project and damaging your business.

How to safeguard your messages from being filtered?

The first and foremost thing to take care of while steering clear of the spam folder is to read the spam filter policies thoroughly. As you don't want your genuine email to go to the spam folder, there are some basics to follow.

Make sure to provide your content only to your subscribers. Even while sending cold emails, warm up your domain beforehand. Always provide ways for receivers to opt out by providing an unsubscribe button.

2. Avoid email bouncing

Never buy or rent email address lists. Build your email database from scratch to avoid sending emails to wrong, inactive or deactivated email accounts to minimize your bounce rates.

3. Quality content

Your content is the highlight of your email message. Do not overuse images and links. Customized content that adds value to your subscribers drives meaningful interaction.

4. Elude HTML-only text

Spam filters are also activated by chaotic symbols, extra tags, or Microsoft Word copy-pasted code. Use customer-friendly models with compatible formats.

5. Activate whitelist

Send your customers a personal email (without any advertisement or marketing links) and ask them to whitelist your domain. If you have been blacklisted from an account, personally communicate with them to get out of there.

6. Run a test campaign

Send emails to various popular email providers like Gmail, MSN, Hotmail, Yahoo, etc., and check your deliverability. You can use a spam filter checker to get a thorough analysis before you send your emails out. It helps in spam email removal if the emails contain any element of spam. Read how SBNRI improved their email deliverability with Mailmodo, leading to a 16% increase in open rates.

If you want to know how to safeguard against different email client's spam filters, you can read these guides:

How to Pass Through the Gmail Spam Filter and Land in Your Customer’s Inbox

How to Pass Through the Yahoo Spam Filter and Land in Your Customer’s Inbox

Outlook Spam Filter: 4 Tips to Safeguard Your Emails Against it

7. Monitor click rates

You can easily degrade your stature and sabotage all your sales efforts if you are not keeping an eye on your email deliverability and how your recipients communicate with your emails. Look out for the content that wins you the attention, and stay clean from those that don't.

Once you get the spam filters on your side, there is no stopping your email marketing success.

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Wrapping up

Spam filters are a crucial element of the email ecosystem. Without them, millions of spam emails would invade the servers, almost preventing the system from working.

Mailmodo is a trusted email service provider that popular brands approve of to elevate sales using AMP emails. Our email experts will help you customize interactive emails that land straight in your customer's inboxes.

What you should do next

Hey there, thanks for reading till the end. Here are 3 ways we can help you grow your business:

  1. Talk to an email expert. Need someone to take your email marketing to the next level? Mailmodo’s experts are here for you. Schedule a 30-minute email consultation. Don’t worry, it’s on the house. Book a meet here.

  2. Send emails that bring higher conversions. Mailmodo is an ESP that helps you to create and send app-like interactive emails with forms, carts, calendars, games, and other widgets for higher conversions. Get started for free.

  3. Get smarter with our email resources. Explore all our knowledge base here and learn about email marketing, marketing strategies, best practices, growth hacks, case studies, templates, and more. Access guides here.

What should you do next?

Thanks for reading till the end. Here are 3 ways we can help you grow your business:


Get smarter with our email resources

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Do better email marketing with Mailmodo

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Talk to an email expert

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