How to avoid spam filters and improve deliverability

In this article, you’ll learn how popular spam filters work, as well as tips and strategies to pass through the filtering and land in the recipient’s inbox when you send campaigns via 

Spam filters are used by companies and email providers to detect potentially suspicious emails and prevent them from reaching the recipient’s inbox. 

By knowing how spam filters work and what they typically look for, you’ll be able to:

  • Improve email deliverability: take actions to increase chances to pass through spam filters. 
  • Increase your sender reputation over time: by consistently sending emails that receive high email scores, you’ll establish a reliable sender identity with email providers. 

How do spam filters work

Spam filters use various criteria to analyze incoming emails and give them an email score which determines whether the email goes in the Inbox or the Spam folder. 

They also analyze content patterns and compare the email to previously blocked ones.

Gateway spam filters

Gateway filters are special servers that receive and review all the emails sent to a company. 

Barracuda is a popular example of a gateway spam filter. Here’s a short overview of how it works: 

  • Checks your sender’s IP address to determine if it’s listed on a blacklist or a list of suspicious senders. 
  • Compares the email content to previously marked spam emails.
  • Check for potential phishing links or viruses in attachments. 
  • Analyzes the email’s intention to see if it contains known spammer tactics.

ESP’s internal spam filters

Email service providers use their own internal spam filters to protect their clients from unwanted emails.

ESP’s spam filters are becoming more and more advanced as they use machine learning to detect spam content patterns. 

The most popular examples are the Google spam filter and the Microsoft spam filter.

In general, they consider the following to determine email score and its trustworthiness. 

  • Email sender reputation
  • Sending emails to recipients who don’t interact with them
  • Sending from a domain or IP address that’s blacklisted
  • Email content 
  • Sending emails without authentication

Desktop anti-spam software

Desktop anti-spam filters are installed directly on the recipient’s computer and can be configured with specific rules to filter incoming emails.

For example, a user can create a whitelist to allow only emails coming from a list of allowed senders or domains to reach the inbox. 

They serve as additional protection after emails have passed the spam filters of the Email Service Provider.

An example of anti-spam software is SpamAssassin. Similar to other spam filters, it additionally checks multiple email elements like:

  • Sending IP reputation
  • Subject line
  • Email headers
  • Authentication
  • Email content and formatting

How to pass spam filters: tips and strategies 

When you send emails, you can’t completely avoid spam filters. But you can improve your chances of getting through by following some tips and guidelines.

If we were to summarize what spam filters have in common, we would find out that they evaluate three main factors:

#1 Email sender reputation

Spam filters are configured to evaluate the sender’s reputation as on of the primary factors based on which it is decided whether the email is spam or not.

Reputation data consists of previous sending volumes, frequency, and recipient engagement.

If your reputation is considered too low to pass, your email will be moved to spam.

With every email you send, you build the sending history and reputation for your domain and IP address. 

Tips to improve sender reputation:


1) Use a clear sender identity

A consistent and recognizable sender identity helps you build reputation and trust with email providers.

Use your full name + brand name in the “From” name field in your email account settings.

The domain in the “From email” field is recommended to match the domain you are sending from.

2) Set up email authentication and DNS settings

Email authentication is a common practice used by ESPs to monitor sender reputation.

It helps the receiving server to verify if the email is authorized to be sent from the sending domain.

Emails that fail authentication checks may be blocked by spam filters. 

There are different settings available to confirm the sender’s identity, such as SPF, DKIM, rDNS, and DMARC. Make sure to set up all authentication records correctly because they work well together. 

Here are the guides to help you learn about SPF, DKIM, DMARC

3) Warm up accounts and be consistent in sending volume

If you suddenly start sending hundreds of emails per day without prior consistent activity or preparation, you are more likely to get blocked by spam filters.

Email warm-up helps in gradually increasing your sending volume to prevent triggering spam filters.

Start email warm-up → 

Consistent volumes in both your actual campaigns and warm-up campaigns contribute to high engagement rates and a positive sender reputation.

Remember: Keep your warm-up running before-during-after sending campaigns


The daily warm-up volume should correspond to the sending volumes of your regular email campaigns. For example, if you typically send around 50-100 campaign emails daily using the same email account, set your warm-up daily limit to around the same volume.

#2 Recipient engagement rate

Along with the sender’s reputation and authentication, spam filters consider how engaged recipients are with your previous emails, meaning the actions they take when they receive them.

If recipients open, forward, or reply to your emails, it’s seen as positive for your email score. 

If recipients ignore or delete your emails even after a few follow ups, sending them more can harm your reputation and reduce the chance to deliver upcoming emails. 

Read tips for improving the response rate in your campaigns.

1) Remove unengaged recipients

In campaigns, you can keep track of engagement statistics and handle recipients who aren’t actively engaging with your emails.

You can create a separate list for these recipients and send them a re-engagement campaign at a later time.

Learn how to filter recipients based on their engagement → 

2) Track high bounce rates

Handling bounced emails correctly is important to keep a healthy reputation. automatically detects bounced emails and stops sending the sequence for that recipient to avoid getting more bounces. 

There’s also a bounce rate protection feature that automatically pauses campaigns that reach the dangerous threshold.

Troubleshoot high bounce rate using this guide → 

#3 Email content

Just avoiding spam trigger words like “free”, “offer”, “sale”, “discount” won’t guarantee your email won’t be marked as spam. 

Spam filters check every component in the email, such as the email text, subject line, links, attachments, HTML code, and the number of images.

Write effective emails with Email AI assistant →

Tips on how to pass the content filtering:


1) Personalize your email text and subject line

By personalizing your subject lines and email content using prospect fields, you’ll demonstrate to spam filters that you’re sending targeted campaigns rather than generic spam emails. 

In email editor, you can insert variables that will be populated with data from prospect fields when the email is sent. 

Learn more about adding custom email variables → 

2) Avoid adding multiple images and links 

If you want to show something or make your email more interesting, try not to include more than one picture, graphic, or video. This will help with your email loading speed . 

Overall, if your email seems normal in formatting, grammar, and structure, it will help with the email score.

3) Don’t add heavy attachments

Spam filters may block your email if they consider the attached file dangerous or if it doesn’t meet file size or type restrictions.

To send a specific file to your recipient, consider using services like Google Drive or Dropbox and insert a link to download the file.

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