Everything you need to know about email deliverability to avoid the spam folder––for good

Email marketing is one of the most successful marketing strategies. According to Statista, it offers a return on investment (ROI) of $45 for every $1 spent. And the Content Marketing Institute reports that 81% of marketers use email marketing.

But just because so many folks use it, does that mean email marketing is easy?



In 2021, nearly 320 billion emails were sent and received each day. Your customers’ inboxes are overflowing, and it’s a challenge to win their attention. On top of that, email service providers (ESPs) have built intelligent algorithms to filter out spam emails.

“Email marketing will not be an overnight success,” says Jigs Reodica, of Klaviyo partner Growth Gurus. “This channel’s power lies in personalization and creating beneficial relationships between the brand and the customers. Thus, it involves creating new connections and nurturing them. At the same time, keep in touch with your current customers through carefully thought-out campaigns.”

“Email marketing will not be an overnight success.”

Jigs Reodica of Klaviyo partner Growth Gurus

So, just sending emails isn’t enough. You not only need an excellent strategy for making sure your emails get into customers’ inboxes to begin with—your emails also need to engage customers.

It might sound like a lot of work, but you can make the process smoother—and get a higher ROI on your email marketing—by learning the basics of email deliverability: a business’s ability to skip the spam folder and get a good inbox placement.

Let’s look at what happens when you send an email and the factors that influence your email deliverability. We’ll also consider several strategies and best practices for improvement.

Here’s what you’ll learn:

Email deliverability basics

Email delivery is the process of sending an email to a recipient’s mail server. But you can’t just send an email and call it a day—you need to get that email into your customers’ inboxes.

To improve your email deliverability, you should know the path an email takes from the sender to the recipient.

The path of an email from sender to receiver.

It’s important to note that your email could be dropped at different stages in this journey.

Here’s why:

Suppressed emails

Some emails will never leave your outbox even when you meant to send them. Some recipients in your email list might have revoked their consent or unsubscribed. Your ESP suppresses these emails, and that means your email list could be much smaller than its initial size.

Bounced emails

Most emails will leave your outbox—but not all of them may reach your recipient’s mail server. Some of these emails could bounce.

There are two types of email bouncing: hard and soft bounce.

A hard bounce happens when the recipient’s email address is invalid. That could happen because they entered the wrong email address in the sign-up form.

To avoid that issue, you could add some basic validation on your sign-up forms. Here are a few options:

  • Email regular expressions to check if the email address is formatted correctly.
  • Check to make sure the user has entered the correct domain.
  • Check to make sure the user’s email address exists.

A soft bounce could happen for several reasons, such as:

  • The recipient’s inbox is full
  • The email server is offline
  • You’ve sent too many emails, and the ESP throttles them
  • The sender’s address is in the recipient ESP’s blocklist
  • The email is too big, or the content violates the ESP’s policies

To avoid soft bounces, you need to be consistent in your emailing volume and careful about your email content.

Automated, guided warming: Companies switching to Klaviyo as their CRM, get access to guided warming, which trains inbox providers to see you as a “good” sender and strengthens your sender reputation.

Emails caught in the spam filter

There are several reasons an email ends up in the spam folder:

  • The recipient marked the email sender’s address as spam.
  • The sender’s email deliverability reputation is low.
  • The subject line or email content raises a red flag.

Unopened emails

Even if your email is delivered to your customers’ inboxes, there’s no guarantee that they’ll open it.

Klaviyo’s 2021 ecommerce benchmark data shows that the average open rate across industries is just 24.37%. That means less than a quarter of your customers will open your marketing emails.

Negative and positive email engagements

As we mentioned before, it’s not enough that a customer opens your email—they need to engage with it. And that engagement can be positive or negative:

Positive engagement

  • Clicking calls to action (CTAs) or other links
  • Adding your address to their contact list

Negative engagement

  • Not clicking on any links
  • Marking your email as spam
  • Unsubscribing

While there are many hurdles a marketing email must overcome to reach its destination, there are also several ways to get past them. Your reputation as a sender, also known as your email deliverability reputation, has a significant role in deciding your email’s fate.

Let’s see what factors affect your email deliverability reputation:

Your sender score—and 3 other factors that influence your email deliverability reputation

Email service providers like Gmail, Yahoo, and Outlook have different algorithms to prevent spam. They all take a few common factors into account.

Here are the main factors that influence your email deliverability reputation:

How various inbox providers determine sender reputation

Sending domain reputation

The human-readable sending domain—also known as the friendly-from domain—follows the @ after the sender’s name. For example, in the email header below, Tripadvisor’s sending domain is mp1.tripadvisor.com.

Sending domain example
Screenshot (email)

Email providers consider the following factors when determining the sending domain’s reputation:

If the ESP finds the sending domain’s reputation unsatisfactory, your emails might end up in your customer’s spam folder. You can check your domain reputation with a free tool.

Sender’s IP reputation

The sender’s IP address is the computer-readable address that emails are sent from. Your ESP may assign the same sender’s IP to multiple businesses. If your IP is in the recipient’s ESP block list or has a bad reputation, your email might end up in spam.

ESPs know that multiple businesses share a sender’s IP, so they might not give a lot of weight to it while calculating your email deliverability reputation. But if you use a shared IP, you need to be careful about your IP reputation. Other businesses’ poor sending practices could affect your deliverability.

In those cases, you may want to go for a dedicated IP. We’ll look at that in detail later.

From address reputation

The from address is the email address that recipients see when interacting with your email. In the email below, the from address is awards.

Example of the from address
Screenshot (email)

If you use a shared sending domain or shared IP, your sending domain and address differentiate you from others. You can use different from addresses for transactional and marketing emails, but it’s a good practice to keep your sending domain consistent.

Message content

ESPs are always alert about scams or phishing emails. They scan an email’s subject line and content to detect anything that looks like spam. Certain words in your email may raise a red flag with spam filters.

You can use tools like Litmus to check for email content issues.

How to improve email deliverability in 6 steps

There are 6 main ways to improve your email deliverability.

  1. Identify your engaged customers and clean your email list
  2. Optimize your email content
  3. Consider a double opt-in process
  4. Segment your email list
  5. Use a dedicated sending IP
  6. Monitor your email deliverability metrics

Let’s look at each one.

1. Identify your engaged customers and clean your email list

If you send multiple emails to unengaged customers, ESPs might flag your emails as spam. That’s why you need to clean your email list periodically. Good list hygiene includes removing all unengaged customers at least once a month.

Regularly cleaning your lists helps you stay away from spam traps. ESPs, internet service providers (ISPs), and anti-spam organizations create spam traps to identify those who buy email lists in bulk.

Along with removing inactive recipients, you also need to identify your most engaged customers. Try designing your email marketing strategies based on engagement tiers.

Sending targeted emails helps reduce email deliverability issues.

“If you’re concerned about email deliverability, it’s critical to clean your email list,” says Spencer Flaherty, internal marketing manager at Klaviyo partner Groove Commerce.

“If you’re concerned about email deliverability, it’s critical to clean your email list.”

Spencer Flaherty, internal marketing manager at Klaviyo partner Groove Commerce

“Not only will unengaged contacts negatively affect marketing metrics, they will also distort your understanding of your audience, making it harder to modify your strategy for the better.”

2. Optimize your email content

Your email content has a huge impact on whether or not your emails reach your customers’ inboxes and are engaged with it, so spending time on your email design will improve email deliverability.

Here’s how you can do that:

Good subject lines

Your subject line will attract customers and improve your email’s open rate. It also affects your email deliverability reputation.

Here are some subject line good practices:

  • Optimize the length of your subject line.
  • Don’t overuse emojis or slang.
  • Try to keep punctuation to a minimum.
  • Use personalization wherever possible.

Check out our 21 tips for great subject lines, which also include some great subject line examples.

Email body

A carefully crafted email body can help you escape spam filters and hit your customers’ inboxes. You need to strike a balance between image and text in your email. Email on Acid suggests at least 500 characters in your email copy.

You also need to include a prominent CTA. It could be a promotional offer, demo, or product recommendation. And including an unsubscribe link is also crucial.

But there are no strict rules that define what makes a good subject line or email body. You need to continually A/B test your emails to figure out what combination works for you.

Mobile-friendly content

Research shows that 63% of marketing emails are opened on a mobile phone. Nobody wants to engage with clumsy emails that don’t show up correctly on their device. So be sure to optimize your email design for mobile.

3. Consider a double opt-in process

A double opt-in process will make sure that your customers are actually interested in receiving your emails. With double opt-in, your customers first sign up using a form and then receive a confirmation email. You can only start sending email messages after the customer gives explicit consent.

The double opt-in process has a couple of advantages, such as:

  • Keeping out customers who only signed up for a promotion or download
  • Filtering out invalid email addresses and unintentional sign-ups

4. Segment your email list

Creating user segments from your main list will help you increase email deliverability. You can segment your email list based on the following factors:

Customer engagement level

You should customize your email volume based on customer engagement. For example, you could send more emails to your most engaged customers and send only occasional winback emails to customers with low engagement.

Customer demographics

Segmenting your email list based on customer demographics helps personalize your emails. Customer demographics include the following factors:

  • Age
  • Location
  • Gender

Capture even more zero-party data from customers with quizzes: Discover how top brands like Paula’s Choice, Fenty Beauty, Doe Lashes, Amika, and Jones Road all use quizzes to gather zero-party data and enhance the impact of their marketing.

Purchase history

Not all of your customers may be interested in the same products, especially if you have a wide product range. Segmenting customers based on past purchases is a great way to increase open rates and engagement.

5. Use a dedicated sending IP

We’ve seen how your sending IP reputation influences email deliverability. In a shared IP setup, many businesses use the same IP to send their marketing emails, but with different sending domain names.

When you share a sending IP with other businesses, any poor sending practices on their end may affect your email deliverability reputation. If you’re a consistently high-volume sender—say, 1 million messages/month—a dedicated IP address will help improve your deliverability.

But not all brands need a dedicated IP, and you should also be aware of their disadvantages:

  • You can’t piggyback on a fellow business’s deliverability reputation.
  • They take time to warm up.
  • You need to send high-volume emails consistently to gain the ESP’s trust.
  • They can be expensive.

6. Monitor your email deliverability metrics

You have a double opt-in process in place, a clean list, and highly engaged customers. You’ve also created a great email campaign and customer segments. Now what?

It’s time for you to monitor and measure the performance of your marketing emails. Pay particular attention to these email deliverability metrics:

  • Open rate: how many of your emails were opened out of the number of emails delivered
  • Click rate: the number of clicks received on your emails against the total number of emails delivered
  • Bounce rate: what percent of your total emails bounced
  • Unsubscribe rate: the percent of people who opt out from your email list

Klaviyo offers several dashboard tools, like the email campaigns trend report and campaign performance report to monitor your email deliverability performance.

4 email deliverability best practices to stay in good standing

Once you’ve achieved good email deliverability, you need to sustain it. With these email deliverability best practices, you’ll be able to maintain good inbox placement.:

Create a single main list

Your main list contains the emails of each customer who opted in for your marketing emails. Maintaining a single main list is a good email deliverability practice for several reasons:

  • It helps you maintain an error-free list of engaged customers.
  • It can prevent customers from getting duplicate messages.
  • It helps you track and organize your email recipients.
  • With a single list, it’s easy to track its growth.

Klaviyo offers a list growth report to track how your main list is growing. You can access your report by going to the Lists & Segments tab and clicking Reports > List Growth.

Klaviyo’s list growth report.

Set up a welcome series for subscribers and new customers

When sending your welcome email series, you may want to differentiate between new subscribers and new customers.New subscribers who haven’t made a purchase yet may be interested in knowing more about your brand and products.

Your new customers may already know about your brand and products but could be looking for information on their recent purchases. You could also send them useful upsell or cross-sell suggestions.

Klaviyo’s customized welcome email series example.

Trigger engagement tracks

Tracking customer engagement is helpful in personalizing email content and volume. You may want to segment your customers based on their open and click-through rates.

You can then create various email campaigns based on your customers’ engagement level, like the one in the image below:

Email marketing campaigns based on customer engagement.

Note: iOS 15 has introduced many privacy changes to limit engagement tracking, which obscures email opening and click-through activities. To navigate these changes, check out Klaviyo’s new features for audience segmentation, revenue attribution, and analytics.

Launch––and abide by––a sunset policy

Continually sending emails to unengaged customers affects your email deliverability reputation and hurts your email marketing. A sunset policy helps you remove the customers who no longer engage with your emails.

Yes, you can try to win back those customers using a winback flow, which can help you attract inactive subscribers by offering high-quality emails and promotions.

But if your customers don’t respond to the winback flow, it’s a good idea to start a sunset flow to remove them from your email list. The sunset flow empowers you to say goodbye to a customer—at least for now.

What is a sunset flow?

A sunset flow is an email automation that lets customers know they won’t be getting emails from you any longer. This is the last step to take before you remove someone from your list as part of regular list cleaning.

Why should you use a sunset flow?

Continually sending emails to subscribers who never open or click them signals to email providers (Gmail, Hotmail, etc.) that you are not a reputable sender.

Eventually, these providers will begin placing your emails in the spam folder.

To avoid this, you must regularly clean your lists. Before giving up on these contacts completely, try to re-engage them using a Winback flow and then offer them a last chance to remain a subscriber via a Sunset flow.

What does a sunset flow look like?

You can create a sunset flow easily in Klaviyo. Here is what it looks like on the backend:

Final thoughts: how to improve email deliverability and avoid spam

Email marketing can be tough due to high competition and email saturation. Email service providers and anti-spam organizations have strict policies against spammers, so you need a great email marketing strategy to ensure your emails land in your subscriber’s inbox.

Your email deliverability reputation decides whether your emails reach your customer’s inbox or spam folder. You can achieve a good inbox placement by:

  • Keeping your email list clean
  • Targeting engaged customers and removing inactive customers
  • Optimizing your email content
  • Tracking your customer engagement
  • Monitoring your email deliverability metrics

To maintain good email deliverability, you need to stick to email deliverability best practices. There’s a lot to it, but with the right tools and information, you can nail email deliverability.

Sign up for Klaviyo’s email deliverability tools, or check out our documentation to learn more.

Back to Blog Home
Get email marketing insights delivered straight to your inbox.
*By entering your email address and clicking Subscribe, you consent to receive marketing emails (such as newsletters, blog posts, webinars, event invitations and new product updates) from Klaviyo from time to time. You can unsubscribe at any time by clicking on the “Unsubscribe” link at the bottom of our emails. For more information on how we process your personal information and what rights you have in this respect, please see our Privacy Policy.
Own your data.
Own your growth.
© 2022 Klaviyo. All rights reserved. Klaviyo and the Klaviyo logo are trademarks or registered trademarks of Klaviyo, Inc. or its affiliates.
Terms and Privacy Manage Cookies