All You Need To Know About Spam Filters

You spent a lot of time, energy, and money crafting the perfect email campaign. You clicked send. And then you got the unpleasant surprise – your email didn’t land in the inbox. It went to the spam folder instead. After all that effort, surely your email should have been delivered successfully, right? Well, not necessarily. You don’t need to be a spammer to get your email sent to the junk folder. In fact, according to ReturnPath, 21% of legitimate commercial emails land in the spam folder or don’t get delivered at all. This is because ISPs have become exceedingly vigilant in ensuring that only relevant emails will land in the inbox. Spam filters are used to ensure that customers are always protected from unsolicited emails.

What are spam filters?

Spam filters are exactly as their name suggests. These are filters that use a list of criteria to evaluate the “spamminess” of an email. The criteria includes the content of the email. Certain phrases and words can signal spam such as ‘buy now’, ‘cash’, ‘viagra’, ‘double your money’, ‘lose weight now’, and ‘last chance to win.’ Excessive punctuation can also be considered spammy language. After evaluation, an email is given a “spam score” which will then determine whether the email will pass through the filter and go to the inbox or if it will get flagged as spam and sent to the junk folder (or not sent at all). In short, spam filters are like TSA personnel. They screen your email, only letting the good ones come in and keeping the bad ones out.

Why use spam filters?

Did you know that 70% of all email around the world is spam? In 2018 alone, around 135.4 billion spam emails were sent per day. Just imagine if a portion of that landed in your inbox. You’ll get buried in irrelevant messages. But that’s the least of your worries. Spam email is generally defined as any solicited email that’s sent in bulk. However, a lot of spam is often malicious, hiding malware or a phishing scam. In 2017 alone, companies lost $676 million because of spam emails from scammers. With spam filters, you don’t have to worry about any of this.

Spam filters explained EmailOversight

Spam filters explained

We’ve already mentioned that spam filters have a list of criteria that they use to evaluate the “spamminess” of an email, one of which is content. But there are other factors to be considered. For example, Gmail and Outlook use subscriber engagement, how the subscriber interacted with previous campaigns, to determine whether your latest email gets in or not. We describe these factors more in detail below.

Good Signals


If a subscriber opens your emails more often than not, this is seen as a positive sign that your latest email is not a scam.


If your subscribers respond to your email by replying to it, this is another good signal. Also, this helps improve your sender reputation.

Not junk

If subscribers are moving your email from the junk folder to their inbox and marking it as “Not Junk”, this is a strong indication that your emails are relevant and good enough to land in the inbox.

Move to folder

Similar to “Not junk”; if people move your emails into one or more of the folders in their inbox, then this is another indication that the recipients place value on your messages. It makes email providers more likely to deliver your future campaigns to the inbox.

Add to address book

If your email address is added by your recipients to their address book, this is considered by email providers as a strong signal that they want to receive emails from you. So, they’re more likely to continue letting your emails land in the inbox.

Bad Signals

Move to junk

If your recipients are moving your emails from the inbox to the junk folder, it is a strong indication to email providers that people don’t want to receive messages from you.

Delete without opening

If recipients don’t even open your email before deleting it, then it sends a strong signal to email providers regarding the worthiness of your campaigns.

All these factors considered together will provide you with two scores:

#1: a score with an individual subscriber – If an individual subscriber is opening your emails and moving them to folders in their inbox, then you’ll get a good score with that particular subscriber.

#2: a score with the email provider as a whole – if many of your subscribers open your emails and move them to folders, then you’ll get a good score with the email provider.

These two scores are weighed together by the email provider when determining whether your future emails are going to be delivered successfully. If, for example, your score with an individual subscriber is pretty high, it doesn’t automatically mean that your emails will always be delivered successfully to that person. It will still depend on your sender reputation with the email provider. If that score is low, then your campaigns won’t get delivered at all, even to that subscriber who engages with your emails.

Avoid Spam Filters

How to avoid spam filters?

Use A Double Opt-In

One of the most commonly recommended email marketing practices is to always get permission from your recipients before adding them to your mailing list and sending them campaigns. To ensure that you get a high quality, permission-based email list, it’s best to use the double-opt-in method. This method requires potential subscribers to confirm their email after they put their email into your online form which is proof that they do want to receive your campaigns.

Use Tools

It’s not enough that you think your email campaign is good enough. You need to make sure that all your time and effort is not wasted. You can do this by utilizing one of the available spam testing tools online. Some of these tools are free like Contactology and Mailing Check. Others are provided by email campaign programs like Mailchimp and Constant Contact. These tools can evaluate how likely your email campaign will pass the spam filters.

Email early

The email addresses on your mailing list can’t wait forever, even if you utilized the double opt-in method. Email addresses can go stale if you don’t send to them regularly. This can lead to unopened emails, unsubscribes, a high rate of bounces, and even spam complaints.

Besides, it’s important that you don’t send large quantities of emails out of the blue. Doing so also triggers spam alerts. Don’t wait until you’ve gathered 1000 subscribers before you start sending your campaign. Instead, you should start sending campaigns to small groups while you build your list.

Use a real reply-to address in your campaigns

We mentioned earlier that email providers consider it a good sign if subscribers respond to your campaigns via email. If you were a spammer, people wouldn’t respond to you. So, if your campaign is getting responses, it tells the email provider that the recipients want to receive messages from you.

This means that when you’re sending out your email campaigns, make sure that you only use email addresses that they can reply to such as or Seeing email addresses like that can encourage your subscribers to respond which improves your sender reputation. On the other hand, using email addresses like discourages responses.

Configure your account to send from your business domain

Make sure that you send your email campaigns from your business domain name, especially if you’re using an email marketing software. The default setting is usually a generic domain maintained by the software provider. A generic domain can trigger a spam alert while a business domain legitimizes your email campaigns in the eyes of email providers. This is because, in general, spammers don’t expend the effort to create business domain names.

Choose body content and subject lines carefully

One of the most commonly used anti-spam techniques is to avoid using “spammy” language. Using capitals, phrases that show urgency, and excessive punctuation in your subject lines and within your email can trigger spam alerts. The same can be said for certain words such as “free”, “cash”, “last chance to win”, “viagra”, and “buy now”.

Watch your text to image ratio

Often, spammers will send emails containing large images with very little text. This is due to the belief that spam filters can’t read their offer in the image. However, spam filters nowadays are alerted when your text to image ratio is high.

Watch your link to text ratio

Another way to trigger spam alerts is to send an email with a link or more with little to no text. Now, that’s not to say that you shouldn’t include a link in your content. It’s a crucial part of your call-to-action. Just make sure that you include a lot of text along with the link to avoid getting flagged as spam.


Spam filters may seem like they’re not your friend, making things more difficult for email marketers to reach their customers. However, they do the opposite. Taking note of anti-spam techniques will increase your open and click-through rates and which will signal to email providers that your future campaigns are worthy of landing in the inbox. In short, spam filters can help improve your relationship with your subscribers. Because of them, email marketers must always deliver highly relevant content that their recipients will engage with which will give your brand a huge advantage over your competitors.

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