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Email Bounce: Everything there’s to know about it

Denys Kontorskyy

Have you ever sent an email only to get an automated note saying it bounced back? If so, you’re not alone, just about everyone has experienced this. In fact, email marketers deal with this all the time.

What is a bounce, and what are the common reasons behind it? What measures can be taken when numerous emails are bouncing?

In this blog post, we dive into the world of email bounces and cover all of these questions. 

What is an email bounce?

An email bounce occurs when an email service provider (ESP) tries to deliver an email to the recipient’s email server, but it’s returned without being accepted, resulting in an automated bounce message to the sender indicating a delivery error. There can be a variety of reasons why an email bounces, but more on this later.

Imagine a small business owner sending out a mass email to promote a new product to a list of 100 potential customers. From that list, one or two emails bounced. Now you might ask, “What’s the big deal?”… And you’re right; chances are two emails are not going to have much of an impact.

However, if, over time, the business expands and the email list grows to 10,000 potential customers, the same ratio of bounced emails would become a much larger problem. A high bounce rate could lead to the business’s emails being marked as spam, resulting in a reduced delivery rate and visibility for future marketing efforts. So it’s essential to keep track of bounced emails and the reasons behind them. This is where an email bounce code comes in handy. 

What’s an email bounce code?

Part of Internet Message Format standards, email bounce codes, as old as emails themselves, are a set of numbers assigned to an undeliverable email message that indicates why the message wasn’t delivered.

Here are two code examples that Microsoft Outlook users or anyone using Azure in their email infrastructure might see:

554 5.2.122:  The recipient has exceeded their limit for the number of messages they can receive per hour. For more information go to… 

452 4.3.1: Insufficient system resources (TSTE). For more information go to…

Generally, codes that start with a 5 indicate that the error is permanent, and those that start with a 4 mean it’s a temporary issue.

With these codes in hand, it’s easier to manage your rate of emails bouncing and make necessary adjustments.

How does email bounce impact email deliverability?

When an email bounces, it sends a negative signal to the email sender’s server, potentially affecting the sender’s reputation and overall deliverability in the future. Additionally, they can negatively impact the recipient’s engagement metrics, such as click-through and open rates, leading to a decrease in email effectiveness.

Knowing what causes an email to bounce can help reduce errors and improve deliverability.

Types of email bounces: The causes and how to handle them?

Bounces can happen for numerous reasons. The simplest approach to understanding and dealing with them is first to categorize them:

Once you understand the bounce type, you can have a deeper look at understanding the reasons and what action needs to be taken to avoid them moving forward.

Soft bounce 

A soft bounce is an undelivered email message that usually indicates a temporary issue related to the recipient’s mailbox or account rather than an issue with the sender. Some of the main reasons why soft bounce happens are:

When the recipient’s inbox is full, the email server will bounce the email back to the sender. This means that whenever you get a bounce message with a code that indicates full inbox as the reason, it still makes sense to keep that email address on your list. Once the inbox is cleaned or extra storage is added, the recipient will be able to receive new emails.

However, be mindful that if you keep getting this error from the same email address after a few delivery attempts in a span of the campaign cycle, chances are the inbox is an abandoned one.

This type of bounce happens when the recipient’s email server is experiencing high traffic volumes. It’s crucial to keep an eye out if there is some kind of a pattern.

If a significant number of emails to a specific email provider (e.g., Gmail, Yahoo, etc.) are being bounced back with the reason “server overload,” it’s likely that the recipient’s email server is experiencing high traffic volumes at a specific time of day or day of the week. To increase deliverability rates to a specific email provider, it’s important to adjust email sending schedules accordingly.

This soft bounce is caused when the email being sent is too large for the recipient’s email server to handle.

Some email clients, such as Gmail and Microsoft Outlook, have a maximum email size limit of 25 MB, while others may have a limit as low as 10 MB. It is essential to keep this in mind and adjust your email size accordingly to avoid soft bounces due to email size.

Similar to server overload, keep an eye out for patterns. One solution to prevent such bounces from a particular ESP is to break large emails into multiple smaller emails or split the content into smaller chunks and send it over multiple emails.

The recipient’s ESP can sort incoming emails into categories based on their content, sender, and other factors. This reduces the amount of spam or unwanted emails but can also lead to legitimate emails bouncing. Here is an example of a bounce message you get from Gmail:

421 4.7.0: Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily blocked. For more information, visit Prevent mail to Gmail users from being blocked or sent to spam.

This code begins with a 4, meaning it’s a soft bounce, but regardless, take heed of any warnings regarding email content. Neglecting such notifications over time can result in a hard  bounce that’ll be a much more severe problem. Once this becomes a hard bounce due to content issues, an ESP will simply block your domain, and getting it unblocked is no walk in the park. 

To avoid such problems, always act promptly by reviewing the content of the email and ensuring that it complies with the recipient’s mail server spam filter rules. If needed, take the initiative and reach out to the recipient’s ESP for assistance and advice on how to avoid your emails landing in the spam folder. 

A simple network outage can also cause a temporary server error, leading to a bounced message. So just resending the email, in most cases, will work.

Hard bounce

Suppose you receive a permanent email delivery failure (hard bounce) notification from a mailbox provider. Unlike in most cases with a soft bounce, resending the message later will not work and will actually damage your sender’s reputation. 

Before resending anything, changes must be made to either the message or the recipient’s address and maybe even to your email infrastructure.

Let’s take a look at some common cases of hard bounces that email marketers face:

If an address has typos, is missing crucial information, or the email account simply doesn’t exist anymore, it’ll result in a hard bounce. To prevent this type of bounce, verifying your email list regularly and removing any invalid email addresses is essential.

Of course, anyone can make a typo in their email address, so an occasional hard bounce due to this is no reason to be alarmed. What is a red flag, though, is seeing that this type of bounce happens regularly during an email marketing campaign. If that’s the case, review how you collect and store your email list. 

Another point to remember is that simply deleting email addresses from your subscriber list is only sometimes the first thing to do. It’s possible that your customer/app user just got a new email address and still wants to get emails from you. So always check if there is a way to remind the person to update their email address. For example, this can be done via an in-app push notification.

Issues with email authentication can often lead to hard bounces. But the truth is that there is no reason to have these types of bounces if the proper steps are taken. Several email authentication protocols can easily be implemented to help prevent these bounces, such as DKIM, SPF, and DMARC. 

If you already have all of the protocols integrated, it’s still possible to get messages that say the bounce happened due to poor compliance with one of the protocols. This usually means that there is some sort of an issue with authentication settings, and the recipient server is blocking delivery from your domain or, even worse, IP address.

The good news is that most of the time, reviewing and adjusting authentication settings will quickly fix the issue. The bad news is that this type of bounce can still happen even after all the settings are corrected. This is because your email domain name has already been picked up by a credible blacklist that an ESP uses. If this is the case, you’ll need to figure out what blacklist it is and make your case with the admins to be removed.

Bonus tip: Make sure you align your sending practices with Google and Yahoo’s new sender guidelines to avoid bounces.

What is a good email bounce rate?

When considering the effectiveness of an email campaign, it’s essential to understand what’s considered a low bounce rate. Most marketing tools have a campaign monitor and some automation that calculates and shows your bounce rate, but it’s still important to know how it’s calculated.

Let’s say you sent 800 emails successfully, while 20 were undeliverable. The bounce rate, in this case, would be:

(20 / 800) x 100 = 0.025 x 100 = 2.5%

This means that for this campaign, you had a slightly high email bounce rate of 2.5%

Generally speaking, for any marketing effort to be successful, you should target an average email bounce rate of less than 2%. By following several key steps, you’ll be able to keep that 2% benchmark and improve the effectiveness of email marketing efforts. 

 With a lower bounce rate, you can expect to see an increase in conversions, higher engagement rates, and a more successful email marketing campaign.

This is a process of sending a confirmation email to each new subscriber, requiring them to confirm their subscription before being added to your email list. This might seem like an unnecessary extra step, but nothing is worse than inactive subscribers. Having valid email addresses that want to get your content will increase conversions and higher engagement in the long run.

Use digital analytics tools to track your bounce rate and identify any patterns or issues that may be contributing to a high bounce rate. Make changes to your email strategy as necessary to improve your results.

Ensure that your emails are formatted correctly, don’t come across as spammy, and that their  content is valuable and not over-promotional. No one likes spammers, so remember that the subject line is the first thing people see! So pay extra attention to what is written there, otherwise, the first thing people will do is search for the “Unsubscribe” button. An excellent method to determine which type of subject lines work best is to run A/B testing and analyze the open and click-through rates of the same email with a different subject.

Avoid email bouncing: the role of email infrastructure.

The success of any email marketing strategy hinges on properly functioning email infrastructure. This is why it’s important to have a solution to monitor and reduce bounce rates.

Mailtrap is an Email Delivery Platform that lets businesses and individuals test, send, and control their email infrastructure in one place. The platform consists of Email Sending and Email Testing products.

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Email Sending provides developers with a stable email infrastructure for email sending and email performance monitoring that comes with high deliverability rates by design.

Our Email Sending comes with two sending options: SMTP for quick and easy integration with any application to start sending in seconds, and Email API for greater flexibility.

Mailtrap Email Sending lets you control your email performance with unique monitoring tools such as helicopter-view dashboards and drill-down reports with detailed stats on mailbox providers:

Furthermore, you can see email categories and extended email history with historical data up to 60 days of email logs, and detailed stats. 


Email Sending is equipped with a robust monitoring system to keep track of bounce rates and provide detailed statistics. The critical alert system that immediately notifies you if your bounce rate has increased is based on extensive cross-industry research conducted by Mailtrap. This research determines the threshold values. Currently, a bounce rate between 2-5% is considered a warning level (yellow), while a bounce rate greater than 5% is considered a critical level (red).

The system helps you stay informed and proactive in maintaining a healthy email campaign. Additionally, you can select specific recipients to receive Deliverability Alerts notifications about them via email or integrate alerts with Slack for convenience.

For example, the above chart displays a sudden increase in bounce rate above the 5% threshold on March 17 and between 21-23, triggering a warning and prompting the user to review this data. This also means that a deliverability alert was sent regarding this email campaign performance, notifying the user of the sudden drop in deliverability metrics. 

Using Email Testing, a safe environment to inspect and debug emails in staging before they are sent to recipients, you can quickly validate your email’s HTML/CSS, check how emails render across different devices, and review raw email data. 


On top of that, Email Testing lets you analyze email content for spam score and if your email server has been blacklisted. 

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In conclusion, understanding email bounces and their causes is crucial for maintaining good email deliverability and avoiding negative impacts on your email marketing efforts. Keeping track of the bounce codes and adjusting your email-sending practices accordingly is essential, whether it’s a temporary or permanent issue.

Using the right tools for regularly monitoring bounce rates and ensuring your email infrastructure is robust and up-to-date can go a long way in maintaining good email deliverability.

Gain further understanding of email bounces: 

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