How Do Spam Filters Work?

Published: 02-Mar-2022
Last Updated: 11-Jan-2022
How Do Spam Filters Work?

Spam filters are rules-based filters designed to automatically sort emails and prevent annoying or malicious emails from reaching users. Email filters grade incoming messages, and if the message's spam score is too low, it fails to pass through to the inbox.

There are two main types of spam filters server-side filters and email client filters. Organizations can also develop spam filters to control incoming & outbound emails that pose a threat to the organization. But we are going to focus on these two main types in this blog post.

Email client filters are what most of us think of when we hear spam filters. Examples of email client filters are Outlook and Gmail. Server-side filters are developed by lesser-known tech companies and have unique systems/criteria. Examples of server-side filters include Symantec Cloud and Spam Assassin.

The exact algorithms, machine learning, and criteria used by these different spam filters are only known to a select few working for America's tech giants. 

Here are a few factors and issues that are known to land your email in the spam folder.

Spam Complaint Rate

Customer complains to a company representative.

There's no debate whether or not spam complaints affect your deliverability. Multiple subscriber complaints identifying an email as spam is one of the strongest signals to ISPs that future emails shouldn't pass through to the inbox. 

Email messages don't have to be obvious spam to be marked as spam. It's common for legitimate messages to receive complaints. 

Legitimate emails are commonly reported as spam because email marketers:

- Force subscribe web visitors

- Send too many emails 

- Send email blasts that aren't relevant to their entire list. 

- Fail to properly segment their list

- Don't make it easy to unsubscribe 

The issues listed boil down to understanding your audience and making an effort to tailor messages to different segments of your audience. 

If you don't create relevant and segmented marketing emails, you're sure to lose subscribers or worse. The worst-case scenario for sending an email is it being marked as spam because this action can affect your ability to send future emails. 

A high spam complaint rate will tank your deliverability rate. Your deliverability rate will tank because email spam filters will redirect your message to the spam folder. 


There are a few simple things to check off when creating an email that will reduce the risk of triggering a spam filter. 

The first is sending email campaigns from a verified email domain. Sending email marketing campaigns from a free email account (your personal Gmail address, Outlook, etc.) could result in your email not making it to the inbox. Sending emails from a verified email domain sends a stronger signal to ISPs that your email is legitimate. 

Another tip is to always address your emails to the recipient's name, instead of addressing the email to their email address. "Hi," is a great way to start an email, for a robot.

For the humans reading this, it's an embarrassing way to start an email. It's too easy not to add this bit of personalization. Most email marketing software makes it simple to quickly add your recipient's name to your email with merge tags. 

Checking off these basic things will increase the likelihood of your email making it to the inbox.

Are you Blacklisted?

A Blacklist (aka a blocklist) is one of the many tools used by email service providers to decide whether or not an email should make it to the inbox. Email blacklists are lists of spam-sending IP addresses and/or domains that are updated in real-time. 

Blacklists are developed by large ISPs (for internal use) and companies that's have built their business around supplying this information to ISPs. Popular blacklist companies include Spamhaus, Spamcop, Invalument, and Barracuda. These companies each create multiple lists that track different information. 

Not all blacklists are created equal. A blacklist doesn't influence the deliverability of your email if it isn't used by major ISPs. That being said, the inclusion of your IP Address on an influential blacklist (like one of the ones mentioned above) will result in plummeting deliverability rates, the closer of your ESP account, and open your organization to lawsuits. 

For more information about the relationship between ESPs and Blacklist companies, check out this blog post from Hubspot

Issues with Code

Man points at an issue with HTML code.

Don't let non-coders code your emails, and that's coming from a non-coder who was paid to "create" HTML/CSS email templates in a previous role. The potential consequences of an errant piece of code are too high.

HTML/CSS issues within an email template can result in spam triggers. If you're unwilling or unable to invest in an employee qualified to code HTML/CSS emails, we suggest investing in a drag and drop email tool. 

Do you Have a Spam Problem?

It's possible you have a spam problem and do not know it. We discussed that the algorithms, machine learning, and criteria used by ESPs to filter spam are proprietary. The criteria and tech also vary from ESP to ESP. 

Here are a few tools you can use to check if you have a spam problem.

Unlimited and Premier CampaignMonitor users can run free spam tests. Symantec Cloud and Spam Assassin, and two email client filters: Outlook 2013 and Gmail. CampaignMonitors' spam check feature allows users to test their email against four different email filters (Symantec Cloud, Spam Assassin, Outlook, and Gmail) before hitting send.

MailChimp users can access an abuse report in the email software's email campaign reporting dashboard. 

Does your email marketing software not have a native spam test or report? Email marketers who are in this situation can use a 3rd party tool like Sender ScoreMail Tester, or Mailgenius to evaluate whether or not they have a spam problem. 

Wrap Up

Spam protection (filters & legislation) exists to protect consumers, and when we punch out we're consumers. When working from home, there might even be some overlap. Most people who call themselves marketers don't purposely send spam messages, but that doesn't mean their messages won't get labeled by email providers or angry customers. Once you figure out if and why you have a spam problem, follow the guidelines above to make your marketing more effective and improve your customer experience. 


What's Next?

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