Why do Emails Bounce? All to Know

Learn why emails bounce and how to prevent delivery issues in the future. Discover common causes, like invalid addresses and formatting errors, and find solutions.
Email bounces can happen for various reasons, from poorly written addresses to formatting missteps that prevent emails from reaching their intended destination.

If you've ever sent or received a bounced email, chances are you were left feeling frustrated and unsure why it didn't work - that's where we come in!

In this blog post, we'll explore the different causes of email bouncing and offer helpful solutions so that you can avoid these issues when sending emails in the future.

Keep reading to learn all about why emails bounce and how to ensure your messages get delivered safely every time.

What is an Email Bounce?

An email bounce is a notification or automatic response generated by an email server when it is unable to deliver an email message to its intended recipient. This bounce-back message is sent to the sender of the original email to inform them that their message was not successfully delivered.

Email bounces happen for various reasons, such as an invalid recipient address, a full inbox, or issues with the recipient's email server, and they help senders understand why their emails didn't reach their intended destination.

Bounce emails typically contain error codes and information to assist the sender in diagnosing and resolving the delivery issue.

What is an Email Bounce Code?

An email bounce code is a numeric or alphanumeric code that provides specific information about why an email message couldn't be delivered successfully. These codes are typically included in your bounce message when an email bounces back. Email servers use these codes to categorize and communicate the reason for the failure to the sender.

Each bounce code corresponds to a particular issue, such as an invalid recipient address, a full mailbox, a server timeout, or other delivery problems.

These codes are standardized, making it easier for email senders to understand the nature of the bounce and take appropriate action, such as correcting the recipient's email address or contacting their email service provider to resolve server-related issues. Understanding these codes can help email senders diagnose and troubleshoot email delivery problems effectively.

Here are some email bounce code examples from different email providers:

Google Workspace (Former G Suite)

  • Bounce Code: 550 5.1.1

This code indicates a permanent failure to deliver the email. The "5.1.1" part specifies that the recipient's address is invalid or doesn't exist. It could be due to a typo or the recipient's email account being deleted.

  • Bounce Code: 452 4.2.2

This code signifies that the email couldn't be delivered due to temporary server issues on the recipient's end. It suggests that the recipient's server is currently unavailable, and the email should be retried later.

Hotmail/Outlook (Microsoft):

  • Bounce Code: 550 5.7.1

This code is often associated with email rejection due to suspected spam or unauthorized sending. It means that the recipient's server has refused to accept the email because it believes the sender is not authorized or the email looks like spam.

  • Bounce Code: 554 5.2.0

This code suggests that the email couldn't be delivered due to a permanent failure. The "5.2.0" part indicates that the message contains content that's not allowed or is considered offensive, and it's been rejected by the recipient's server.

  • Bounce Code: 550 5.4.1

This code indicates that the email delivery was aborted because a recipient's email address doesn't exist or is no longer valid.

  • Bounce Code: 421 4.4.0

This code is a temporary failure indicating that the recipient's server is experiencing issues or is busy. The email delivery should be retried later when the server becomes available.

Types of Email Bounces

Hard Bounce

A hard bounce occurs when an email you send can't be delivered, and it's a permanent issue. Unlike soft bounces, where you might try sending the email again later, resending won't work with hard bounces, and it can harm your sender’s reputation.

To fix hard bounces, you usually need to make changes. These changes could involve adjusting the message, fixing the recipient's email address, or even tweaking your email setup.

Now, let's check out some common situations that lead to hard bounces:
Invalid or Non-Existent Email Address
When there are mistakes in an email address, missing information, or the email account no longer exists, it results in a hard bounce. To prevent this, checking and removing invalid email addresses from your list is vital.

If someone makes a typo in their email address, it's not a big deal. However, if you see this happening a lot during your email campaigns, it's time to review how you manage your email list.

Sometimes, deleting an email address isn't the first thing you should do. The person might have a new email address but still wants your emails. Look for ways to remind them to update their email, like an in-app notification.
Poor Email Authentication
Email authentication problems often lead to hard bounces. But the good news is, that you can usually prevent these bounces by setting up proper email authentication protocols like DKIM, SPF, and DMARC.

Even if you've set up these protocols, you might still get bounce messages due to authentication issues. This typically means there's a problem with your authentication settings, and the recipient server is blocking your emails.

Sometimes, your email domain could end up on a credible blacklist like Barracuda, SpamCop, or SORBS used by email service providers. If that happens, you'll need to contact the administrators to get your domain removed from the list.

Soft Bounce

A soft bounce is when an email can't be delivered, but it's usually not a big problem on the sender's side. Instead, it's often a temporary issue with the recipient's email system.

Here are some common reasons for soft bounces:
Full Inbox
This happens when the person's email inbox is completely full, so there's no space to receive new emails. If you get a bounce message saying the inbox is full, don't worry too much.

Keep the email address on your list. Once they clear some space or get more storage, they should be able to get your emails. But if you keep seeing this error from the same address after a few tries, it might mean the inbox is abandoned.
Server Overload
Sometimes, the email server of the person you're sending to is very busy and can't handle more emails at that moment.

If you notice a pattern of emails bouncing with a "server overload" reason, it's best to adjust when you send emails to improve your chances of them getting through.
Email Size
If the email you're sending is too big for the recipient's email server to handle, it can result in a soft bounce. Different email services have size limits, so be aware of those limits and adjust your email size accordingly. Check out our blog post about Gmail sending limits.

If this happens a lot, you might consider breaking up large emails into smaller ones.
Email Filtering
Some email services automatically sort incoming emails into categories, which can sometimes cause legitimate emails to bounce. Pay attention to any warnings you receive about your email content.

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.

Ignoring these warnings can lead to more serious problems later on.

Review your email content to prevent this and ensure it follows the recipient's email service rules. Contact their email service provider for guidance on avoiding the spam folder if necessary.
Temporary Server Error
Sometimes, a temporary network problem can cause an email to bounce. In most cases, simply resending the email should work.

Pending Bounce

A pending bounce, also known as a deferred bounce, is a situation where an email you've sent isn't immediately delivered to the recipient and is temporarily put on hold by the recipient's email server.

This delay happens because the recipient's email server encounters a temporary issue that prevents it from accepting the email at that moment.

Instead of bouncing the email back to you immediately, the server attempts to deliver it again later.

Here's why a pending bounce might occur and how you can address it:
Temporary Server Issues
One common reason for a pending bounce is that the recipient's email server is experiencing a temporary problem. This could be due to server maintenance, high traffic, or other technical issues.

The server temporarily holds your email until it can successfully deliver it.
In this case, there's not much you can do except wait. The recipient's email server will further attempt to deliver the email when the issue is resolved.

However, if the problem persists for an extended period, you might consider reaching out to the recipient through an alternative method to inform them of the situation.
Some email servers use a technique called greylisting to combat spam. When an email is sent from an unknown sender, the recipient's server may temporarily reject it with a "try again later" message.

Legitimate email servers will attempt to resend the email after a delay, while many spammers won't bother. This helps reduce spam.

If you encounter greylisting, you typically don't need to take any action. Your email server should automatically retry sending the email, and it should eventually get through to the recipient once it's recognized as a legitimate sender.
DNS Issues
DNS (Domain Name System) problems on either the sender's or recipient's side can sometimes lead to pending bounces. If the recipient's email server can't verify the sender's domain or has trouble resolving it, it might delay the email.

Ensure that your DNS settings are correctly configured on your email server. If you suspect DNS issues on the recipient's side, you can notify them of the problem so they can address it.
Improve your email deliverability with Unfiltered.ai

What Kind of Emails Can Bounce?

Marketing Emails

Email bounces can be harmful to your email marketing efforts.

They happen when an email is undeliverable to the recipient's inbox and are categorized into two main types: hard bounces (permanent issues) and soft bounces (temporary issues).

High bounce rates can harm your sender's reputation, impact email deliverability, and reduce the effectiveness of your marketing campaigns.

Here’s how to avoid bounces on your email marketing campaign:
Maintain a Clean Email List
A clean email list is the cornerstone of effective email marketing. Over time, email addresses may become invalid or abandoned, leading to hard bounces.

By regularly cleaning your list, you identify and remove email addresses that are no longer active, reducing the risk of sending to addresses that won't deliver.

Email verification tools can help automate this process by checking the validity of each address on your list.

Cleaning your list also prevents spam traps, which are email addresses created to catch spammers. Sending to these addresses can severely damage your sender's reputation.
Implement Double Opt-In (Confirmed Opt-In)
A double opt-in process adds an extra layer of verification to your email list.

After someone subscribes, they receive a confirmation email and must click a link or take another action to confirm their subscription.

This ensures that the email address provided is valid and that the subscriber genuinely wants to receive your emails by reducing the chances of fake or mistyped email addresses entering your list.

Double opt-in not only reduces bounces but also leads to a more engaged and interested subscriber base, as only those genuinely interested will complete the confirmation step.
Use a Dedicated IP Address
If your email marketing volume is substantial and consistent, consider using a dedicated IP address for sending your emails. This means your email reputation is solely based on your sending practices.

While shared IP addresses can be cost-effective, they may expose you to the sending habits of other users, which can affect your reputation.

When using a dedicated IP, following email best practices rigorously is crucial to building and maintaining a positive IP reputation. Additionally, some email service providers may require you to gradually warm up a new dedicated IP address to establish trust.

One-To-One Emails

Email bounce in one-to-one emails refers to the scenario where an email you've sent to an individual recipient encounters an issue that prevents it from reaching the recipient's inbox.

Instead of your email being delivered as intended, it's returned to you with an error message or notification, indicating that the delivery was unsuccessful.

This can happen for various reasons, including typos in the recipient's email address, issues with the recipient's email server, full mailbox, or restrictions on the recipient's side. Email bounces for one-to-one emails are important to address promptly, as they indicate that your message did not reach its intended destination.

By understanding and resolving the cause of the bounce, you can ensure that your one-to-one communication is effective and reaches the recipient without interruption.

Cold Emails

In cold emails, an email bounce refers to an email sent to a recipient with no prior relationship or communication with the sender that encounters an issue preventing successful delivery.

It means that the cold email did not reach its intended recipient and was returned to the sender, often accompanied by an error message or notification.

This type of bounce can occur due to various reasons, including:

  1. Invalid email address: The recipient's email address may be misspelled or nonexistent.
  2. Recipient's email server issues: The recipient's email server may be temporarily down or experiencing technical problems.
  3. Email filtering: Aggressive spam filters or email security measures may block the cold email, preventing it from reaching the recipient's inbox.

Now, let's explore three best practices for cold email campaigns:
Personalization Beyond the Name
When sending cold emails, personalization is your key to breaking through the initial barriers of unfamiliarity. Including the prospect’s name in the intro is not enough. Successful personalization involves researching the prospect’s background, company, and interests.

Use this information to craft a highly targeted message acknowledging their individuality and highlighting how your product or service can benefit them. Mentioning recent accomplishments of their company or industry trends can show that you've invested time and effort in understanding their needs.

When the prospects feel that you've taken the time to personalize your message, they are more likely to view your email as relevant and not just another generic outreach attempt.
Warm-Up Your Email Domain:
Your email domain reputation plays a significant role in email deliverability. Email service providers (ESPs) like Gmail or Outlook assess the sending domain's reputation when deciding whether to deliver an email to the inbox or spam folder.

Sending a sudden large volume of cold emails from a previously inactive or unestablished domain can trigger alarms and lead to your emails being flagged as spam. To avoid this, it's crucial to warm up your domain gradually.

Start by sending a limited number of emails to recipients within your existing network or those who have explicitly opted in.

Over time, gradually increase the volume of cold emails. This warming-up process builds trust with ESPs, increases your sender reputation, and reduces the risk of your emails bouncing or being marked as spam.

If you want to avoid this extensive work and improve your deliverability faster and better, consider using an email warm-up tool, like Unfiltered.ai
Respect Opt-Out Requests
One of the cornerstones of ethical and compliant email marketing, including cold email campaigns, is providing recipients with a clear and easy way to opt-out or unsubscribe from your emails.

Honoring these opt-out requests promptly is not just a legal requirement in many jurisdictions but also essential for maintaining your sender's reputation.

When someone requests to opt-out, it's a clear signal that they're not interested in your emails. Continuing to send emails after an opt-out request can lead to high bounce rates, spam complaints, and damage to your sender's reputation.

To implement this best practice effectively, ensure that your opt-out process is straightforward and that opt-out requests are processed promptly to prevent further communication with recipients who wish to discontinue receiving your emails.

What is a Good Email Bounce Rate?

A good email bounce rate is typically considered to be below 2%. This low rate is desirable because it indicates that you send emails to valid and active recipients, ensuring your messages reach the right audience.

A high bounce rate can harm your sender's reputation, impacting email deliverability and potentially causing your emails to be marked as spam or blocked. Also, reducing bounce rates can lead to cost savings by optimizing your email campaign expenses.

An email warm-up tool like Unfiltered.ai can help lower bounce rates by gradually establishing a positive sender’s reputation with email service providers. These tools allow you to send emails incrementally, starting with a small volume and increasing it over time.

By mimicking natural email behavior, warm-up tools reduce the likelihood of triggering spam filters. They also monitor recipient engagement, helping you identify which emails engage recipients and which might be bouncing or going unopened.

Additionally, warm-up tools assist in list cleaning and identifying invalid or inactive email addresses, reducing bounce rates and improving email deliverability.


Clearly, understanding and monitoring email bounce is an essential step to creating successful email campaigns. Email bounce can be hard or soft, resulting from incomplete address data, and can either go undetected or cause a long-term disruption to deliverability.

By grouping emails specifically into one-to–one or marketing, an organization can better monitor bounces by type and create a plan of action accordingly.

Even with the best efforts at mitigating bounce rates, however, it can still be difficult to achieve that “ideal” rate of success with all types of emails.

Fortunately, with the use of an email warm-up tool such as Unfiltered.ai, more personalized campaigns can be created, and sending servers won't have trouble delivering our emails anymore.
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