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Why Do Emails Bounce? 9 Key Reasons Behind Your Undelivered Messages

Email is the backbone of many businesses and personal interactions. However, not every email sent reaches its intended destination. Sometimes, they bounce back, leaving the sender puzzled and the recipient unaware. But what does it mean when an email bounces? Why does it happen, and how can it impact your communication or marketing efforts?

This guide delves into why emails bounce, explaining what they are, why they occur, and how they can be categorized. We also explore 9 common reasons behind email bounces and provides insights on how to address them. By understanding and addressing the causes of email bounces, you can improve your email deliverability, protect your sender reputation, and ensure your messages get to where they need to! 

What Does It Mean When An Email Is Bounced?

When an email is bounced, it means that the email you attempted to send was not successfully delivered to the recipient’s inbox. This can occur due to a variety of reasons, and it’s a signal that something has interfered with your message before it can be read by your intended recipient.

Email bounces are categorized into two main types: hard and soft bounces.

Hard Bounce

Hard email bounces are a permanent failure of email delivery. This typically occurs when the recipient’s email address is invalid or non-existent. This could be due to a typo in the email address, the email account being closed, or the domain no longer existing. Hard bounces are a serious issue because they can negatively impact your sender reputation. It’s important to promptly remove these addresses from your email list to maintain your email deliverability health.

Soft Bounce

On the other hand, a soft bounce means a temporary failure of email delivery. This can happen due to reasons such as the recipient’s mailbox being full, the email server being temporarily down, or the email message being too large. Unlike hard bounces, soft bounces don’t necessarily require immediate removal from your email list as they may resolve over time. However, if an email address continues to soft bounce over multiple email sends, it may be worth considering its removal from your list.

There are many things you do to reduce bounce rates such as double opt in, maintaining good email list hygiene, authenticating your emails, and only sending to subscribers who are interested.

9 Reasons Why Your Email Bounces

Email bounces can be frustrating and detrimental to your email marketing efforts. Understanding the reasons behind these bounces can help you take corrective measures and improve your email deliverability. 

Here are 9 common reasons why your email might bounce:

1. Invalid Email Address

An invalid email address is one of the most common reasons for an email to bounce back. This typically results in a hard bounce, indicating a permanent email delivery failure issues. There are several scenarios in which an email address may be considered invalid:

Incorrect Email Address

This usually happens when the email address has a typo or error. It could be as simple as a misspelt domain name (like “” instead of “”) or a misplaced character. In such cases, the email server cannot find the intended recipient, and the email bounces back.

Non-Existent Email Address

Sometimes, the email address does not exist. This could be because the address was made up or input incorrectly at the time of collection. For instance, a user might have entered a fake email address to access a gated resource without wanting to provide their real email.

Deleted Email Account

If a recipient has deleted their email account, all emails sent to that address will bounce back. This could happen if a person leaves a company and their work email is deactivated or if a user decides to close an email account.

Invalid email addresses are a serious concern because these bounced emails can negatively impact your sender reputation. Email service providers track the number of bounces you get, and a high bounce rate due to invalid addresses can lead to your emails being marked as spam or even to the suspension of your account.

2. Full Inbox

Another common reason for email bounces is a full inbox on the recipient’s end. When an inbox has reached its storage capacity, it cannot accept any more incoming emails, causing any new incoming messages to bounce back. This is typically classified as a soft bounce, as it’s a temporary issue that can be resolved when the recipient clears some space in their inbox. Try resending to the bounced address later.

A full inbox can occur for several reasons:

  • High Email Volume: Some users receive a large volume of emails daily, and if they don’t regularly manage their inbox, it can quickly become full.

  • Limited Storage Space: Some email providers offer limited storage space. If the user has used up all their available space, new emails will not be delivered until some space is freed up.

  • Inactive Email Account: If an email account is inactive and not being checked or managed, the inbox can fill up over time, causing new emails to bounce back.

3. Server Issues

Server issues are another common cause of email bounces. If the recipient’s email server is down, undergoing maintenance, or temporarily unavailable for any reason, it won’t be able to receive your email, causing it to bounce back. This is typically classified as a soft bounce, as it’s a temporary issue that should resolve once the server is back up and running.

Server issues can occur for a variety of reasons:

  • Scheduled Maintenance: Email servers, like any other online service, require regular maintenance. During these maintenance periods, the server may be temporarily unavailable.

  • Unexpected Downtime: Servers can also experience unexpected downtime due to technical glitches, power outages, or other unforeseen issues.

  • Network Problems: Sometimes, the issue isn’t with the server itself but with the network. If there’s a problem with the recipient’s internet service provider or a network outage, the server may be temporarily unreachable.

While you have no control over the recipient’s server status, understanding that this is a possible cause of email bounces can help you better interpret your bounce reports. If you notice a sudden spike in soft bounces, it could be due to a temporary server issue affecting multiple recipients. 

4. Email Too Large

The size of your email is one of the reasons that could impact its deliverability. If an email is too large, either due to heavy attachments or extensive content, it may be rejected by the recipient’s email server, resulting in a bounce. This is because most email servers have a maximum size limit for emails, and any email exceeding this limit is automatically bounced back.

Here are some factors that can contribute to large email size:

  • Heavy Attachments: Attachments, particularly high-resolution images or large files, can significantly increase the size of your email. If the combined size of your email and its attachments exceeds the recipient’s email server size limit, the email will bounce.
  • Extensive Content: Emails with a lot of text, images, or complex HTML can also be large in size. While these emails may not typically exceed server size limits on their own, combined with attachments, they could potentially cause a bounce.
  • Embedded Media: Embedding media such as videos or high-resolution images directly into the email can significantly increase its size. It’s generally better to link to this content rather than embedding it directly to email large files.

5. Blocked Sender

If the recipient or their email server has blocked your email address or domain, your email will not be delivered and will bounce back. This is typically classified as a hard bounce, as it’s a permanent delivery failure until the block is lifted.

There are several reasons why your email address or domain might be blocked:

  • Marked as Spam: If a recipient marks one of your emails as spam, their email server may block all future emails from your address or domain.

  • Blacklisted: If your IP address or domain has been blacklisted due to suspicious activity or a history of sending spam, email servers that use these blacklists will block your emails.

  • Server Policies: Some email servers have strict policies and may block emails from certain domains or IP addresses that don’t meet their standards. This could be due to a variety of factors, such as your email content, sending practices, or sender reputation.

6. Incorrect DNS Entries

Incorrect DNS entries, particularly your MX (Mail Exchanger) records, can lead to email bounces. MX records are a type of DNS record that specifies the mail server responsible for accepting emails on behalf of your domain. If these records are not correctly set up, other email servers won’t know where to deliver emails sent to your domain, causing them to bounce back.

Here are some common issues with DNS entries that can lead to an email bounce:

  • Missing MX Records: If your domain doesn’t have any MX records, other servers won’t be able to deliver emails to your domain.

  • Incorrect MX Records: If your MX records point to the wrong server, emails sent to your domain will be delivered to the wrong place and likely bounce back.

  • Low TTL Values: The TTL (Time to Live) value of your MX records determines how long they are cached by other servers. If the TTL value is too low, other servers may not have enough time to deliver emails before the MX record expires, leading to bounces.

7. Poor Sender Reputation

Your sender reputation plays a crucial role in whether your emails reach their intended recipients. If your IP address or domain has a poor reputation due to previous spammy behavior, email servers may reject your emails, causing them to bounce. This is typically a hard bounce, as it’s a permanent delivery failure until your reputation improves.

Your sender reputation is determined by several factors:

  • Spam Complaints: If recipients frequently mark your emails as spam, it can harm your sender reputation.

  • Email Bounces: A high bounce rate, particularly hard bounces, can negatively impact your sender reputation.

  • Spam Trap Hits: If your emails are frequently sent to spam trap addresses (email addresses specifically set up to catch spam), it can severely damage your sender reputation.

  • Low Engagement Rates: If your emails are rarely opened or interacted with, it can negatively affect your sender reputation. Monitor your engagement rates and test changes to improve this key deliverability factor.

8. Recipient’s Email Server Rejects Email

Certain email servers have stringent rules and may reject emails that do not meet their specific criteria. These criteria can vary widely from server to server and can include factors such as the email’s content, its format, the sender’s reputation, or the presence (or absence) of specific technical elements. If an email does not meet these criteria, the server may reject it, causing it to bounce.

Here are some common reasons why an email server might reject an email:

  • Spam-Like Content: If your email contains content that the server’s spam filters deem suspicious, such as certain keywords, excessive use of capital letters or exclamation marks, or a high image-to-text ratio, the server may reject it.

  • Lack of Email Authentication: Email servers often check for email authentication protocols like SPF, DKIM, and DMARC. If these are not properly set up, the server may reject the email.

  • Poor Sender Reputation: If the sender’s IP or domain has a poor reputation, the server may reject emails from that sender.

  • Non-Compliance with DMARC Policy: If the sender’s domain has a DMARC policy and the email does not align with this policy, the server may reject it.

9. Auto-Reply or Out of Office Reply

While not a traditional bounce, auto-replies or out-of-office replies function similarly in that your original email is not immediately read by the recipient. This can occur when a recipient has set up an automatic response to incoming emails, indicating that they are currently unavailable to read or respond to emails.

Here’s how it works:

When you send an email to a recipient who has an auto-reply or out-of-office reply set up, their email server automatically sends a response back to you. This response typically informs you that the recipient is currently unavailable, and it may provide additional information such as when they will be back or who to contact in their absence.

While your original email is not rejected or bounced in the traditional sense, it’s not immediately read by the recipient, which can delay communication. However, unlike a traditional bounce, your email remains in the recipient’s inbox and can be read when they return.

Time To Fix Email Bounces!

Understanding these reasons can help you take steps to reduce your email bounce rate, improve your sender reputation, and increase your email deliverability. Regular monitoring and maintenance of your email list, along with best email practices, can go a long way in ensuring your emails reach their intended recipients.