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How to bypass the toughest
spam filters: the ultimate

(plus tips to make sure your emails look great each time)

333 B

emails sent every day1

1 in 6

don’t get to the recipient’s mailbox2

4 in 5

are never opened3

1 in 20

may not render correctly4

Stack the odds in your email’s favor

Use our all-in-one checklist to get them to their destination the first time, every time

Consistently a top channel for marketers, and with an average ROI of $44 for every $1 spent5, email remains one of the most effective and efficient ways to reach your audience. It gives you complete control over the timing, design and messaging, without being subject to unpredictable swings in market-driven advertising costs or changes to search algorithms.

However, there are no guarantees that your emails actually make it to your recipients, or that those emails look as you intended in their inbox. But, if you follow the best practices in this checklist, the chances of getting through and displaying properly is significantly higher.

How do spam filters work?

When spam filters first hit the scene, they were fairly rudimentary — filtering was mostly based on keywords that in theory, indicated an email was likely spam. Since then, a lot has changed to improve how email providers keep unwanted emails out of your inbox.

The introduction of reputation metrics like sender scores and authentication mechanisms like SPF (Sender Policy Framework) and DKIM (Domain Keys Identified Mail) signing, have further helped mail providers judge whether an email is likely spam, or whether the sender was authorized to send from the domains used.

In recent years, email providers have layered on behavioral metrics that measure how people engage with emails. Senders of emails that get lots of opens and clicks, score much better on the spam scales than senders whose emails get deleted, marked as spam or sent directly to trash.

Email deliverability testing

A word of warning though — before you hit send, be sure to test! Even if you follow the checklist below, test for issues before sending to the forty-thousand-or-so people receiving your message. There are countless tools available (paid and free) to identify potential spam issues and preview your email on a variety of devices and email clients. Check out the resources list at the bottom of the checklist for useful links.

How to avoid spam filters when sending emails: a checklist

This comprehensive checklist contains several advanced items (marked with an asterisk*) that require technical knowledge or assistance.

If you’re using a third party mailing service like HubSpot (which takes care of most of the technical aspects of sending email), then not everything is applicable. However, if you’re building email into your own software, you’ll want to make sure your development team ticks all the boxes.

Set up

your infrastructure

Email services and their spam filters look at these basic infrastructure configurations to validate you’re sending email on behalf of your domain name. Most of these things work in the background, and once they’re done it generally means you won’t need to make any further changes to your email sending set up.

Set up an SPF (Sender Policy Framework) record on your domain*

Turn on DKIM (Domain Keys Identified Mail) signing for your messages*

Make sure the IP address you’re sending from is not on a blacklist (see links at end)*

If you’re using a shared IP address to send the email, be sure to use one with a high sender score

If you’re using your own dedicated IP address for the first time, “warm it up” by sending a small amount of emails, and ramp it up bit by bit to earn the trust of large email providers before sending a huge blast*

Set up valid reverse DNS records (also known as PTR records) for the IP addresses that point to your domain*

If you can, send all your messages from the same IP address*


emails that engage

The days of spam filters with hard and fast rules are gone. Email providers are now looking at behavioral signals, and using artificial intelligence to reward email senders that get high engagement (opens, clicks, replies) and punish senders with low-engagement emails. With this shift in how spam filters work, ask yourself, “Would I open this email?”


Send useful, relevant content that your users want to receive and interact with

Provide an HTML and plain text version of the email

Allow people to view your email in a web browser if it doesn’t render properly in their email client

Keep your subject line relevant and accurate – avoid clickbait

Spell check your email

Avoid all caps and exclamation points

Exclude spam words (e.g. weight loss, pharmaceuticals, free, etc)

Avoid excessive copy or keyword stuffing

Do not include links to questionable sites or spammy content

Don't mix different types of messaging in the same email (e.g. don’t put a promotional message in a billing notification)

Links in the message body should be visible and easy to understand. Recipients should know where they’re going when they click links


Make sure the email can be read without loading images – some email clients require the recipients’ explicit permission to load images from unrecognized senders

Use ALT text in your images

Host your images on a CDN (Content Delivery Network) to ensure fast loads around the world*

Use only absolute links (as opposed to relative links) for images*

Avoid large images – compress them if possible

Don’t use CSS-specified background images – older versions of Outlook don’t recognize them*

Steer clear of interactive elements – video, javascript (Tip: Instead of a video embed, link a video thumbnail image with a play button overlay)

If you need to use video, try a GIF, but beware if it’s too large it may take too long to load, and therefore may not be seen

Don’t embed forms

Link to files instead of adding them as attachments


Code the structure using HTML tables. For more complicated layouts, you should nest tables to build complex structures*

Use valid, standards-compliant HTML in your HTML emails*

Make your emails 600-800 pixels maximum width, so they appear in full in the preview pane of email clients

Avoid overly complex CSS – use this guide for compatible styles*

Make sure all your CSS is inline*

Use CSS media queries to make the email adapt to different screen sizes*

Only allow standard web fonts that are available in popular browsers and email clients, such as Arial, Verdana or Georgia

Keep text easy to read – avoid hidden text or small fonts



Email is very much like print publishing – once you hit send, there’s no undo button. That’s why it’s so important to allow time for testing, to make sure you get it right the first time.

Test your email in the most popular email clients like iOS Mail, Gmail, Android Mail and Outlook

Test on a variety of devices – phones, tablets, laptops and desktops

Use a tool like like Mail-tester.com to see how your email scores on popular spam filters

Consider third party email testing services to speed up or automate the process (list at end)


metrics and patterns

Once you’re live and sending emails, keep an eye on the following to ensure your emails have the best chance of getting through.

Watch your email analytics to see what demographics and types of emails have the most engagement

Watch your unsubscribes for patterns, and proactively adjust your sending time and lists to prevent above-average unsubscribes

Keep your bounce rates low

Monitor the sender reputation of your domain and IP address

Keep up-to-date on email legislation, spam filter rules and technology

Bonus resources

Here are some great tools and resources to help your next email get to a real human.

Download a PDF of this checklist
and save it for later


This Spam Filter Checklist is brought to you by Hubspot and TinyMCE

The HubSpot CRM platform makes it easy for your entire company to work together — from marketing, to sales, to customer service. Each hub is powerful alone, but they're even better together.

Learn about how HubSpot can help your business grow better

TinyMCE is the world’s most trusted, adaptable and scalable rich text editor component. Companies like Hubspot rely on TinyMCE to help their users create rich and expressive emails within their email platform.

Learn about how TinyMCE’s WYSIWYG email editor can enhance your app

1 Statista
2 Validity
3 Constant Contact
4 Assumes emails viewed in Outlook may have some display issue. Email market share breakdown: Litmus

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