Even the best email marketing campaigns can flop. You might craft a stellar message, select the perfect target audience, and nail the send time, only to find out later that your emails bounced. Emails can bounce for a number of reasons, but the biggest problem is that you might not find out until it’s too late. This blog will help you understand why emails bounce and implement expert tips to improve inbox delivery and avoid bounce issues. Inframail's email infrastructure can be a valuable tool for achieving these goals. By understanding why emails bounce, you can take steps to avoid issues and improve email deliverability for your next campaign. 

Table of Contents

• What Is an Email Bounce?

• Why Do Emails Bounce?

• How to Improve Your Bounce Rate

• Start Buying Domains Now and Setup Your Email Infrastructure Today

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. Still, it’s returned without acceptance, resulting in an automated bounce message to the sender indicating a delivery error. There can be various 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 will not have much of an impact.

Bounce Volume

If the business expands over time 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, keeping track of bounced emails and their causes is essential. This is where an email bounce code comes in handy.

Understanding Email Bounce Codes

Part of Internet Message Format standards, email bounce codes—as old as emails themselves—are numbers assigned to an undeliverable email message that indicate 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. 

• 452 4.3.1: Insufficient system resources (TSTE). 

Bounce Codes

Generally, codes with a 5 indicate that the error is permanent, and those that begin with a 4 mean it’s temporary. With these codes, managing your email bounce rate and making necessary adjustments is easier.

Why Do Emails Bounce? The Email Bounce Types You Need to Know

You can cluster bounces into two main categories: hard bounces and soft bounces. 

• Soft bounces occur when a temporary issue prevents your emails from getting delivered. 

• Hard bounces happen when there’s a permanent issue with providing your email. 

Whether you’re seeing soft or hard bounces, there are many reasons why a bounce might occur—and whether and how you take action depends on what’s caused the bounce in the first place. That’s why generalized advice like “if you’re seeing a hard bounce, do X.

Soft Bounce Causes

If you see a soft bounce, do Y” can be pretty misleading. Instead, you’ll want to look at what has caused your emails to bounce (those bounce codes are handy for that!) and then tackle the root cause.

Email Bounces and Deliverability: What’s the Big Deal? 

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. They can negatively impact the recipient’s engagement metrics, such as click-through and open rates, decreasing email effectiveness. Knowing what causes an email to bounce can help reduce errors and improve deliverability.

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.

Bounce Rate Calculation

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 you had a slightly high email bounce rate of 2.5% for this campaign.

Bounce Rate Goals

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:

• An increase in conversions

• Higher engagement rates

• A more successful email marketing campaign

Related Reading

• Why Are My Emails Going To Spam
• Email Deliverability Rate
• Email Monitoring
• Email Deliverability Issues
• Email Quality Score
• Bounce Rate in Email Marketing
• How To Avoid Email Going To Spam
• SPF or DKIM
• How To Check If Your Emails Are Going To Spam

Why Do Emails Bounce?

Soft bounces happen when an email gets a temporary failure when trying to reach its destination. If a transient failure delays your email or keeps it from being delivered, mailbox providers will return a soft bounce. Soft bounces indicate that an issue is temporary, and sending the email again in the future might be successful. Bounce codes that indicate a soft bounce start with the number 4.X.X. What transient errors might lead to a bounce? Let’s look at a few common causes of soft bounces.

Mailbox Is Full

Most mail providers limit mailbox size to a certain point. Gmail gives its users 15GB of free storage, while Yahoo! provides its users with 1TB of storage space. Once a user hits that limit, their mail provider won’t accept new messages, so your messages will bounce. Here’s the bounce message you’ll receive from Google if you’re sending mail to a recipient who’s reached their storage limit. smtp;452 4.2.2 

Over Quota Bounce

The email account that you tried to reach is over quota. Please direct the recipient to https://support.google.com/mai... - gsmtp. Because the recipient can free up storage anytime, your next email attempt might be successful—a classic soft bounce.

Abandoned Inbox

If you continue to see the same error message over multiple sending attempts, there’s a chance you’re trying to reach an abandoned inbox, and you might want to remove the recipient from your mailing list.

Delays or Temporary Issues on the Receiver Side

Mailbox providers can experience temporary issues or a heavy load of traffic that can prevent your emails from getting delivered. Let’s look at this bounce from Apple, smtp;451 4.7.1 Service unavailable – try again later. This bounce tells us that messages are not being accepted right now, but you can deliver the same message successfully when you try again later. We occasionally see this for sends happening at the ‘top of the hour’ when many other senders around the world are also sending aggressively to Apple, and Apple's inboxes simply get overwhelmed with processing the incoming mail. If your email service provider handles bounces well, they’ll spot a bounce like this one and automatically retry sending your email after a few minutes (so that you don’t have to do it manually).

Content-Related Soft Bounces

Mailbox providers might choose to bounce individual emails if your content looks suspicious. Even though these are soft bounces (with bounce codes that start with a 4.X.X), you should take all notifications that hint at an issue with your content or your overall sender reputation seriously.

Content-Related Block

If you don’t act, these bounces will likely be upgraded to a hard bounce—or even a full ISP block. Don’t let that happen. Here’s what a content-related soft bounce might look like smtp;421 4.7.0. Our system has detected that this message is suspicious due to the nature of the content and/or the links within. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mai... for more information,  gsmtp

Spammy Content

With this bounce, Gmail makes it clear that it isn’t happy with the content of your email. So it’s time to investigate: Are you using any language that might come across as spammy? 

Spam or Reputation-Related Soft Bounces

In addition to email content, mailbox providers use other insights to determine whether you’re a trustworthy sender. If they’re suspicious of your sending reputation, they might decide to bounce your emails: smtp;421 4.7.0 [TSS04] Messages from 0.0.0.0 temporarily deferred due to user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes This is Yahoo telling you they received a high number of spam complaints from emails originating from your domain, which could now affect deliverability for all messages from this sending IP. You should immediately look at what might be causing them.

Hard Email Bounces: The Causes and Solutions

Hard bounces happen when a permanent issue prevents your email from getting delivered. When mailbox providers send you a hard bounce, they tell you that changes to the message, your infrastructure, or the message destination must be made before the email can be delivered. Resending the message in its current form isn’t successful—and attempting to do so might hurt your sender's reputation. That’s why, in general, you shouldn’t keep sending emails to recipients who hard-bounced before you investigate what caused the bounce and implement a fix. When an email hard-bounces, you’ll see a bounce code that starts with a five (5.X.X.) 

Here are a few common reasons why an email might hard bounce:

Domain or Email Address Doesn’t Exist

You can’t deliver email to addresses that don’t exist. If you’re trying to do so, you’ll receive a bounce that might look like this one from Apple: smtp;550 5.1.1 <example@icloud.com>: user does not exist. If this happens, you’ll want to remove the email from your list. Some ESPs handle this step for you automatically. At Postmark, we’ll automatically handle this bounce for you and deactivate email addresses that don’t exist to prevent future sending to invalid inboxes.

Invalid Addresses

While you should take hard bounces seriously, receiving an occasional hard bounce for invalid emails isn’t unusual. It can quickly happen when a customer makes a typo when adding their email address to a form. If you regularly see a lot of hard bounces of this type, that might indicate larger issues with your list collection practices. For mailbox providers, that’s a sign of a sketchy sender. So, if you see a lot of invalid email addresses bouncing, take a closer look at how these addresses were collected in the first place.

Email Authentication Issues

With DKIM, SPF, and DMARC, you have a toolkit of clever email authentication standards available to help you set rules for who’s allowed to send email using your domain—and what to do if someone who’s not an authenticated sender is trying to send email on your behalf. If an email doesn’t comply with the authentication policies of the sending domain, a mailbox provider might choose to bounce that message, like Comcast is doing here: smtp;550 5.2.0 Message rejected due to DMARC. Please see http://postmaster.comcast.net/smtp-error-codes.php#DM000001. If you’re spotting a bounce code that hints at issues with DMARC with your legit email, you’ll need to review your authentication settings for SPF and DKIM to ensure that your emails are aligned with the DMARC policy. If you don’t, this can hurt your overall sender reputation. Gmail, for example, regularly turns DMARC-related bounces into a full ISP block, preventing all your emails from getting delivered. Ouch.

Spam or Reputation-Related Hard Bounces

We’ve seen examples of spam and reputation-related soft bounces above, but depending on the severity of the issue, similar issues might also cause hard bounces. It’s also fairly common to see a soft bounce first—the mailbox provider is still investigating and might let future emails go through—before upgrading to a more serious hard bounce if the sender isn’t taking action. Let’s check out this example here: smtp;550 5.7.1. Our system has detected that this message is likely suspicious due to the very low reputation of the domain being sent. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mai... for more information. - gsmtp

Reputation Block

A very similar bounce from Gmail is classified as a soft bounce (bounce 421 4.7.0). While the soft bounce is temporary, Gmail has made up its mind when issuing this hard bounce: Your sending domain has a low reputation, so Gmail is now permanently rejecting your messages. You might also see a hard bounce if you’re listed on a significant block list. In this example, Outlook has permanently rejected your email because you’re listed on a block list: smtp;550 5.7.1 Service unavailable, MailFrom domain is listed in Spamhaus. To request removal from this list, see https://www.spamhaus.org/query... (S8002) [#.eop-nam02.prod.protection.outlook.com] 

It’s no surprise that Outlook is hard-bouncing your emails. If this happens, you’ll have to work with Spamhaus to fix the issue and remove your listing.

Related Reading

• DMARC vs DKIM
• Importance Of DMARC
• What Is a Soft Bounce Email
• Email Deliverability Checklist
• What Affects Email Deliverability
• Why Is Email Deliverability Important
• Email Bounce Rate
• Fix Email Reputation
• Improve Sender Reputation
• Email Hard Bounce
• Email Deliverability Tools
• Email Deliverability Best Practices
• Best Email Domains

How to Improve Your Bounce Rate

When fixing email deliverability problems caused by bounces, the first thing to do is monitor bounce rates. This means keeping an eye on the total number of bounces and the rate of bounces (bounces divided by emails delivered). It’s hard to give firm benchmarks on what bounce rate is fine vs. when you need to worry. If you send important transactional emails like password resets or payment failure notices to existing users, even a few bounces can be troublesome.

10% Threshold

But if you’re collecting email addresses at an event for your newsletter, you can expect to see typos and invalid email addresses that need to be removed. No matter what emails you’re sending, your bounce rate should never exceed 10%. Seeing a bounce rate close to or above that 10% threshold is a sign that your emails are seriously wrong. But don’t just look at the total numbers. Sudden changes or spikes in bounces can indicate an issue, too—and if you spot them early, you can fix them before they do more damage.

Clean Your Lists to Remove Invalid Addresses

Another effective way to reduce bounce rates is to clean your email lists periodically. Over time, subscribers change email addresses, and some even become inactive. Removing addresses that are no longer valid or that consistently cause bounces helps improve your email deliverability. Use automated tools to identify and purge these addresses. Also, consider segmenting your lists to categorize subscribers based on activity levels and engagement. This helps in tailoring content that is more likely to be welcomed (and less likely to be ignored).

Manage Bounces Actively Before They Impact Deliverability

Bounces can be a regular part of sending emails, especially if you’re working with many subscribers. Managing bounces actively helps prevent them from negatively impacting your sender's reputation and future deliverability. Use features in your email platform—like SendGrid's bounce list management—to automatically remove addresses that consistently cause hard bounces. Set rules for how many times you will attempt to resend emails that soft bounce before considering removal.

Use Confirmation Emails to Prevent Typos

One of the best ways to prevent bounces is to confirm email addresses before adding them to your list. Always send a confirmation email that requires new subscribers to verify their email address. This extra step helps prevent typographical errors and confirms the subscriber's interest.

Monitor Bounce Types (Not Just Totals) 

We’ve learned that there are many reasons why an email might bounce—and some are more severe than others. So don’t just look at the number of bounces; pay attention to what kind of bounces you’re getting.

Prioritize Hard Bounces

While you might not have the time and resources to investigate every single bounce, there are a few things you should prioritize. As a general rule of thumb, hard bounces are more serious than soft bounces, so you’ll want to pay close attention to the former. You’ll always want to take content, spam, and reputation-related bounces seriously.

Prioritize Hard Bounces

Whether those come in a soft or a hard bounce, they are the most profound indicators of more significant issues with your deliverability. Our SMTP Field Manual can help you understand why you might see inevitable bounces—and how to fix them. If a specific email address keeps bouncing, remove it from your mailing list. If you don’t, the mail server could broadly block your mail. 

Start Buying Domains Now and Setup Your Email Infrastructure Today

Inframail is revolutionizing cold email infrastructure with unlimited inboxes at a single flat rate. We provide:

• Microsoft-backed deliverability

• Dedicated IP addresses

• Automated technical setup to help agencies, recruiters, and SDRs scale their outreach efforts efficiently

Automated Email Authentication Setup

Inframail automates SPF, DKIM, and DMARC setup to help you reach the inbox. These technical configurations authenticate your emails and prove to ISPs that you are a legitimate sender. The fewer emails that bounce, the better your sender reputation. The better your sender reputation, the more emails reach their destination.

Dedicated Email Servers for Each User

With Inframails cold email infrastructure, each user has their own dedicated email server. This means that your deliverability is not impacted by the actions of other users on the same server. Instead, you get dedicated resources to improve your performance and help your emails reach the inbox.

Priority Support

Inframails support team is available 16 hours a day, and we prioritize tickets from our customers. If you run into a problem, you can solve it quickly to minimize any impact on your email deliverability. 

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• Soft Bounce Reasons
• Check Email Deliverability Score
• Soft Bounce vs Hard Bounce Email
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