How Do Spam Filters Work?

In 2022 email marketing still remains the easiest and most effective way to communicate with clients and prospects, and for a reason: with an astounding ROI of 4,200%, email marketing generates $42 for every $1 spent!. This is why email marketing is still widely used by companies.

On average, 83% of B2B companies communicate with a newsletter for their content strategy. 40% of marketing managers define email as a key strategy for their success. HubSpot

How do Spam filters work?

It is, therefore, essential to optimize emailing strategy, especially deliverability. Among elements that impact email deliverability there are certifications, contact lists but also spam filters. We have already spoken about some of these subjects in previous articles, don’t hesitate to check our blog to discover how they can impact your communication strategies.

In this article, we will focus on spam filters. Now more sophisticated than ever, they have become the real enemies of your email marketing campaigns. As soon as you start sending large amounts of email you’re exposed to spam filters.

According to ReturnPath, almost 21% of emails sent by professionals end up in the spam folder, even though they are legitimate.

But what are spam filters? How do they work and how can you avoid them?

Discover our MailSoar experts’ advice to avoid spam filters and boost your email deliverability.

What are spam and spam filters?

Spam is an unrequested and irrelevant email sent to a list of recipients. It can be unsolicited advertisements or fraudulent messages, particularly in the context of scams, phishing attempts or to spread computer viruses.

For example, imagine that you bought a contact list for a specialized company. At first sight this list appears like an excellent prospect pond for your company. You obviously want to contact them to present your company and your services. But since the recipients did not allow you to contact them, your emails will be filtered as spam.

Why is your email identified as spam?

There are two ways to report an email as spam:

  • By the recipient

    The report is done by the recipient by using the “report” button on its email service provider. This information is sent directly to the server, enhancing the spam filter.

  • By Spam Filters

    Spam filters, unique to each email service provider, like Gmail, Yahoo, Outlook, establishing spam placement criterias that we will analyze in the following section.

However, it sometimes happens that spam filters identify your email as spam even if your email is relevant and expected by the recipient. We will see how these filters work to understand how to avoid them.

How do spam filters work?

Spam filters are using multiple criteria to determine if an email is spam or not. All incoming emails are verified by spam filters. They analyze and classify emails taking into account spammers’ practices.

When an email’s score goes above  a certain level, it will be identified as spam and directly rejected into the spam folder.

Spam filters are specific to each email service: an email can pass Yahoo’s spam filters but be rejected by Google’s. This is due to servers that individually determine their criteria list to identify spam. These lists are constantly evolving, but some of them are important to remember:

  • Ip Address

    if your IP address is associated with spam sending or the use of non-reliable ESPs in the past, your IP address can be “blacklisted” resulting in spam landing. Coding: A bad code, the misuse or overuse of tags can alert spam filters. It is therefore important to clean its HTML code to avoid unnecessary tags for example.

  • Coding

    A bad code, the misuse or overuse of tags can alert spam filters. It is therefore important to clean its HTML code to avoid unnecessary tags for example.

  • Content and Layout

    There are no specific rules about content or pictures in email marketing. It is necessary to make sure that your content is relevant to the recipient and the subject

Do you encounter spam issues in your email marketing campaigns ?

Our MailSoar experts can help you fix your deliverability issues

How to avoid being identified as spam?

How to avoid being identified as Spam?

We will now give you some advice to avoid being mistaken as spam for your upcoming email marketing campaigns.

Use a safe emailing platform

Do not use your own emailing software like Gmail, Microsoft Outlook or Yahoo to send your different emailing campaigns. These services, although very efficient, are not suitable for email marketing. You will be limited by unsubscribe links, measuring KPIs, and ROI, and also by the amount of emails sent each day.

We advise using reliable and safe email service providers such as MailChimp, SendingBlue, and FlowMailer.

For example, one of our clients used Gmail to send emails and contacted us for help. With our support their email deliverability increased by 300%. Discover their case study here.

Clearly define your email’s subject

Since the subject line is one of the first things we see in our mailbox, it is important to work on it. It plays a very important role in the open rate of your communications, but spam filters also focus on it.

It is, therefore, very important to take care of it and avoid certain practices in order to avoid landing in spam purely because of the subject.

  • Do not overuse capital letters, exclamation marks or any other special characters: they will make your email go directly into spam.
  • Be careful not to use “spamwords”, words associated almost 100% with spam. For example: ‘Get Paid’, ‘Money Back’, ‘Cash’, ‘Dollar’, ‘Profit’,  might seem to be attractive words, yet  they are frequently used in scams  so are generally identified as spam.
How do Spam filters work?

Clearly identify yourself

It is important to keep the same email address. It will allow your recipients to recognize your sender name, not be reported as spam, and also enhance your address’ reputation. This last point is crucial during your score calculated by spam filters.

Maintain a balance between pictures and text

Since spam filters can’t read pictures, if your email is only or mostly composed of pictures, spam filters will identify you as a spammer trying to bypass the filters.

It is good to find the right balance between text and pictures to illustrate your content without arousing suspicion. Our experts recommend on average a ratio of 60% text and 40% pictures in your communications.

Campaign timing

Spam filters are made to identify bulk email and put them directly into spam. To avoid this problem do not send your campaign emails all at once, try to spread them out in installments. This will allow you to analyze hours that work best for your campaigns while also avoiding the spam folder!

Add a unsubscribe link

How do spam filters work?

Unsuscribe links have been mandatory since 2004 after the CAN-SPAM law in 2003. If you don’t integrate it in your emailing campaigns ISPs are more likely to blacklist your email address. In this case all of your future campaigns will be identified as spam.

Moreover, if you add an unsubscribe link, recipients are more likely to unsubscribe themselves instead of reporting the email. It is therefore very important to display this link.

Forget about attachments

Do you want to attract spam filters’ attention? Then go ahead and add attachments to your emails. In case you didn’t know, attachments and spam filters don’t really get along. As a matter of fact attachments can contain viruses so spam filters will automatically mark your email as spam.

If you need to attach a file to an email we advise you to add it directly in the body of the text if possible, especially for pictures and graphs, or add a download link redirecting to a landing page for example.

In addition to not being considered as spam, your email will be lighter, and as a result faster, to open. It will be easy to track your link’s impact with tags or your landing page’s metrics.

Clean your contact list

Remove contacts that have little or no engagement with your campaigns. Even if it reduces the size of your contact list, it will enhance your sender reputation. Your address list and the way it interacts with your communications has a great impact on your reputation and your email deliverability. The higher your deliverability, the less likely your email is to be considered as spam.


To sum up, spam filters allow you to identify spam by taking into account different criteria. However, a bad email strategy can cause your email to land in spam even though they are legitimate.

How to avoid spam filters

If you need help for your 2022 email marketing campaigns, MailSoar can help you implement the best solution depending on your marketing goals. Our team of experts is used to managing the ongoing deliverability of massive senders from all industries.

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