Why do emails bounce? 6 ways to reduce email bounce rates for better deliverability

Email marketing
September 22, 2023
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You’ve carefully planned your email marketing campaign.

You’re sure that your subject line will drive opens, that the content is thoughtful, and that the CTAs are well-placed.

You press send.

And then you get a bounce-back alert.

It turns out, your email didn’t even land in someone’s inbox. Or—worse—it missed a lot of people’s inboxes.

All your hard work was for naught. Now you have to figure out where it went wrong.

Bounced emails can sabotage your most thoughtful, effective marketing strategies, especially if the number of bounces is high or rising over time.

The success of your email marketing campaign depends on your marketing emails actually reaching recipients’ mailboxes. So, the nitty gritty details of deliverability, sending reputation, and bounce rates matter.

What is a bounce-back email?

A bounce-back email is an email you receive from an email provider telling you the email you sent could not be delivered. You receive a non-delivery report (NDR)—an error message that explains why the email bounced back.

Image shows a non-delivery report from gmail.

Image source: writer’s inbox

What are the types of email bounces?

There are two types of email bounce, and both can have a negative impact on your deliverability rate.

Hard bounce

A hard bounce happens when an email can’t be delivered for a permanent reason, such as a misspelled email address. Klaviyo, the platform that powers smarter digital relationships, automatically suppresses any email address that hard bounces.

Soft bounce

Soft bounces are temporary and result from issues like a full mailbox, email server being down temporarily, or a change in email ID. These can be resolved as the server continues to send the email automatically. If an email soft bounces more than 7x in a row, Klaviyo suppresses the address.

However, soft bounces can become hard bounces if the issue on the recipient end is not resolved.

Why do emails bounce?

A variety of factors could cause a failed email delivery, ranging from temporary issues that resolve on their own to permanent problems.

Here are some common reasons why emails bounce:

Invalid recipient email address

If the bounce-back message you get refers to a “non-existent email address,” chances are, the issue is an invalid email address.

There are several possible reasons for an invalid email address:

  • The user accidentally misspelled their email address.
  • The user deliberately gave you a false email address—especially common when you offer a free item or a significant discount in exchange for an email address.
  • The user moved on to a new email address, and is no longer using this one.
  • The user gave you their corporate email address and they no longer work for that organization. In that case, the IT team probably blocked their email address.
  • The sign-up form was abused by a bot.

Even a discount as big as 20% could prompt users to type a fake or rarely used email address into the form. Apparel brand Sivana offers this large discount in exchange for signing up to receive marketing emails, but they use a double opt-in process to protect from fake email addresses. (More on double opt-in processes below.)

Image shows a form from Sivana offering 20% off if a user enters their email address.

Image source: Sivana

Pro tip: Proofread your email lists, and keep your lists clean, too.

Invalid sender email address

This kind of bounce happens when the email address you’re sending from is misspelled or doesn’t exist.

Pro tip: Double- and triple-check the details for typos (e.g., the spelling of your “from” address) when sending marketing emails.

Full inbox

People get hundreds of emails a day, and let’s face it: Not everybody keeps their inbox clean. Add the fact that mailboxes have limited capacity, and you’ve got the perfect conditions for a bounced email.

Email content or attachments exceed size limits

A related reason your email might bounce back is if the length or size is larger than the per-message limit. And it’s not just the size of your email’s content that can hurt you—if your attachments are too large, emails will bounce, too.

Pro tip: Compress your file sizes using compression software, or split a series of images into an email flow.

Image shows an error message from Gmail that says “Note: there were errors attaching your files. Send this message without these attachments?”

Image source: writer’s inbox

Email server issues

Your recipient’s email server may have crashed, it may be under maintenance, or it may even be overloaded.

Pro tip: Try re-sending the email 1-2 more times. If it’s still not getting delivered, consider it a hard bounce.

The recipient has added an auto-reply

If your recipient is OOO and has enabled an auto-response, the email provider sends that response your way immediately after your email hits their inbox.

This is good news: You know your email has arrived.

But if you keep getting an auto-reply, chances are the contact has abandoned that email address.

Pro tip: If you get a bounce-back email from the same address for a few months, consider removing that contact from your list.

The email is blocked by spam filters

Emails can get blocked or filtered for all sorts of reasons:

  • The email address in the “from” field is not recognized.
  • The subject line has misspelled words or all caps.
  • The subject line seems suspect, with purely promotional language or words or phrases that could trigger a spam filter, like “FREE MONEY!!!”
  • Recipients report content as spam to the email provider. This leads to poor sender reputation (more on this next).

Pro tip: In order to sidestep those spam filters, follow subject line best practices.

Poor sender reputation

If you don’t have a strong sender reputation, email providers may filter out your emails.

These 4 factors affect your sending reputation the most:

  • Negative engagement with your messaging: complaints, delete-before-reads, and unsubscribes
  • Lack of engagement: no opens, clicks, or forwards
  • Spam trigger terms in your emails and subject lines
  • Content links reported as abusive: most common when you use third-party links or image hosting services

“Any of the factors above tell the inbox provider that the recipients are no longer interested or possibly did not even opt in to begin with,” explains Tonya Gordon, manager of deliverability and compliance strategy at Klaviyo.

“This results in filtering more and more of those emails to the spam/junk folder, bouncing it due to spam-like activities, or—even worse—‘dropping it on the floor,’” Gordon adds. “This means the message did not bounce or even get delivered to the inbox or spam folder.”

Any of the factors above tell the inbox provider that the recipients are no longer interested or possibly didn’t even opt in to begin with. This results in filtering more and more of those emails to the spam folder, bouncing it due to spam-like activities, or–even worse–dropping it on the floor. This means the message didn’t bounce or even get delivered to the inbox or spam folder.
Tonya Gordon, manager of deliverability and compliance strategy

Pro tip: Read our deep dive on email deliverability to learn exactly how to improve your sender reputation.

What is an email bounce rate and how do you calculate it?

Calculate your bounce rate by using this equation:

number of emails bounced / number of emails sent

For instance, if you send 20K emails and 200 of them bounce back, your calculation would be:

200 / 20,000 = 0.01

To understand your bounce rate as a percentage, simply multiply the result by 100:

0.01 x 100 = 1%

What is the average email bounce rate benchmark?

While average bounce rates vary depending on the health of your email marketing program, Klaviyo’s product analytics team recently determined that businesses using Klaviyo for more than 90 days have average bounce rates under 1%.

Image shows a chart indicating bounce rates under 1% for Klaviyo users who’ve been using the platform for more than 90 days.

Image source: Klaviyo

For newer Klaviyo users, who are more likely in the process of building their lists and segmentation strategies, email bounce rates are a little higher—hovering around 2% on average.

Image shows a chart indicating bounce rates between 2-3% for Klaviyo users who’ve been using the platform for less than 90 days.

Image source: Klaviyo

6 ways to reduce high email bounce rates

If you find yourself with high email bounce rates, take heart. There are several ways to reduce them.

1. Make sure the email addresses you’re sending to are real, consented profiles

It’s crucial that your subscribers are real, and that they’ve directly consented to your marketing.

In order to ensure this:

  • Never buy a list.
  • Never message email addresses who have not opted in to receive communications from your brand.
  • Limit the amount of on-site giveaways or contests that aim to collect email addresses. People may use fake or rarely checked email addresses just to get the incentive.
  • Create a separate list for email addresses you acquire through those on-site giveaways or contests.
  • Focus on other ways to acquire subscribers, like free shipping or other discounts.
  • Segment your lists to send to engaged subscribers that are actively interested in your brand.

2. Enable double opt-in to ensure your new subscribers are real—and really want your emails

A double opt-in method requires a subscriber to confirm their subscription via email before being added to your list.

If you use Klaviyo, lists are set to double opt-in by default.

A double opt-in process protects you from:

  • Well-intentioned subscribers who accidentally typed their email incorrectly
  • Spam bots that find your form and flood it with fake email addresses
  • Folks who are curious enough to add their email address to your form, but not actually interested enough to confirm

Any of the above can increase your bounce rates and hurt your sender reputation.

Here, see how CBD brand Joggy includes the first step of their email opt-in process at the bottom of their homepage.

Image shows colorful bottles with Joggy’s logo on them, and at the bottom right, circled in red, is a field for users to enter their email address.

Image source: Joggy

After a visitor enters their email address, they get this automated email, asking them to confirm their subscription.

Image shows a black background with Joggy written in bright white letters. The text reads: “You’re almost in. Please click on the button below to confirm your subscription + ensure our emails don’t end up in your spam folder.” Below that text is a line highlighted in green reading “Yes, I want to subscribe.”

Image source: Joggy

3. Clean your email lists regularly

It’s important that you clean your email and SMS lists regularly. In addition to removing any fake or misspelled emails, you need to get rid of the unengaged ones as well. This will not only weed out any profiles who shouldn’t receive your messaging, but also decrease your spam complaints—improving your sender reputation over time.

Pro tip: Set aside time to clean your lists at least once a month, and make sure you follow all the correct steps.

4. Remove soft bounces

Once you identify your soft bounces, proactively removing them can boost your deliverability performance. You can exclude or even suppress email addresses that have recently soft bounced from your campaigns.

High bounce rates can harm your sender reputation, so it’s crucial to manage and update your list accordingly.
Natalie Sullivan, former retention marketing manager
Avex Designs

Natalie Sullivan, former retention marketing manager at Avex Designs, recommends addressing chronic bounces. “Keep an eye on email addresses that consistently bounce,” she says. “High bounce rates can harm your sender reputation, so it’s crucial to manage and update your list accordingly.”

In order to find these profiles, create a segment that identifies chronic soft bounces. Include anyone with at least 4 bounced emails in the last 90 days and who isn’t suppressed.

Image shows a product screen from Klaviyo where users can remove subscribers to whom emails have bounced at least 4x in the last 90 days.

Image source: Klaviyo

Pro tip: Klaviyo automatically suppresses profiles that soft bounce 7x in a row, but each time an email bounces, it still affects your deliverability. That’s why it’s important to exclude or suppress soft bounces.

5. Segment to show spam filters you have useful content

Segmentation can improve your deliverability because the more personalized an email is, the more likely a recipient is to open it and actually read it. Regularly opened emails show email spam filters that you’re safe, improving your sender reputation.

You can segment your lists according to:

  • What subscribers want to hear about
  • How often they want to hear from you
  • How long they’ve been a subscriber

Apparel brand Buck Mason sends subscribers to this preferences page once they click on a “Manage Preferences” link at the bottom of an email. The form is quick and easy to fill out, and responses provide powerful insight into what kind of emails will actually interest subscribers in the future.

Image shows a screen from Buck Mason that allows subscribers to say how often they want to hear from the brand, and to choose what they’re interested in hearing about.

Image source: Buck Mason

6. Send emails at a regular cadence

To maintain strong open rates and a positive relationship with your subscribers, align your sending schedule with your segmentation strategy. If you send too often to unengaged profiles, their lack of engagement will hurt your sender reputation. Meanwhile, if you don’t send enough to engaged customers, you may be missing out on revenue.

To determine the best sending schedule:

  • Segment your subscribers into tracks based on how engaged they are.
  • Take into account how long someone has been a subscriber, as well as how (and how often) they engage with your emails.
  • Send more frequent emails to the most engaged subscribers.
  • Send less frequent emails to less engaged subscribers.
  • Consider sunset flows or re-engagement campaigns for unengaged profiles.

Gordon shares this recommendation: “If you’re sending to less engaged subscribers, keep the volume below 15% of your most engaged subscribers. This will help your reputation remain positive.”

Jewelry brand La Kaiser sends this email to a subscriber who hasn’t interacted with marketing messages in a while, inviting them to update their preferences and offering a discount.

Image shows an email from La Kaiser, a jewelry brand, asking “Do you still love us?” It offers the subscriber a chance to manage their preferences, opt out of emails, or get 15% off if they buy something.

Image source: La Kaiser

Pro tip: Visit the Klaviyo help center for a deep dive on how to create a sending schedule based on email engagement.

Tweak your email strategy to improve your deliverability rate

Email marketing is the cornerstone of digital marketing and continues to drive more growth and ROI than most other channels. An integral aspect of the strategy is maintaining your sender reputation.

Bookmark this guide so you can come back to it as needed. It will help you always keep your bounce rate as low as possible—and your sender reputation and deliverability rates high.

Email bounce-back FAQs

What are some common email bounce-back codes?

Auto-reply bounce codes provide more specific information about why you have an undeliverable email message. These codes help you troubleshoot the reason for the delivery failure and determine the best course of action. Some common bounce codes include:

  • SMTP 421: Your message was temporarily deferred by the recipient server. This is usually a result of too many connections in a short timeframe or too many messages.
  • SMTP 450: Your message was not delivered because the other user mailbox was not available. This can happen if the mailbox is locked or is not routable.
  • SMTP 451: Your message simply failed, often because of a far-end server problem.
  • SMTP 452: There isn’t enough system storage to send your message. Your message is deferred until storage opens up.
  • SMTP 550: Your message failed because the other user’s mailbox is unavailable or because the recipient server rejected your message.
  • SMTP 551: The mailbox your message was intended for does not exist on the recipient server.
  • SMTP 552: The mailbox your message was sent to does not have enough storage to accept your message.
  • SMTP 553: Your message was not delivered because the name of the mailbox you sent it to does not exist.
  • SMTP 554: This is a vague message failure response that can refer to any number of problems, either on your end or with the recipient server.

Is there a limit to the number of bounce-back emails I can have before it affects my deliverability?

There is a limit to the number of bounce-back emails you can have before it starts to affect your email deliverability. Email providers typically monitor bounce rates as an indicator of sender reputation and email quality. Consistently high bounce rates can signal that your email list is outdated, poorly maintained, or potentially used for spamming.

Should I be concerned about an occasional bounce-back email?

Occasional bounce-backs are common and usually not a cause for major concern. However, if you consistently experience high bounce rates or encounter specific bounce errors, it’s important to investigate and address the underlying issues to ensure reliable email delivery.

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Annie McGreevy
Annie McGreevy
Senior editorial writer
Annie McGreevy is a senior editorial writer at Klaviyo, where she researches, interviews and writes about how businesses of all sizes can better leverage their owned marketing channels to succeed on their own terms in the current economic environment. Previously, she was a ghostwriter for thought leaders in the payments industry and taught writing to undergraduate students for more than a decade at The Ohio State University. Also a creative writer, her fiction and essays have appeared in Electric Literature, The Los Angeles Review of Books, Nouvella Books, and elsewhere. She lives in Ohio and loves the cold weather, hiking, and a good Zoom background.
Drew Sanocki
Drew Sanocki
Drew is an ecommerce veteran. He started and sold his own online retailer, then served as CMO at Karmaloop and Teamwork. He explains his data-driven growth strategies at Nerd Marketing and works with direct-to-consumer brands through his agency Growth Engines.