85+ Email Bounce Codes: The Definitive Guide

Meet Josh, an email marketer who just sent out a big campaign.

Everything seemed perfect until the bounce notifications started rolling in.

Josh noticed both soft and hard bounces and felt overwhelmed.

What do these bounce codes mean? Why did his emails fail to reach their targets?

Understanding these codes can help Josh – and you – figure out what’s going wrong and how to fix it.

In this guide, we’ll break down what these bounce codes mean, why they happen, and how you can fix them.

What is an email bounce? Soft vs hard bounce

Have you ever sent an email and wondered why it didn’t get through?

That’s what we call an email bounce.

It’s when your email can’t be delivered to the recipient’s email account, and it gets sent back to you with a bounce message.

There are two main types of email bounces: soft bounce and hard bounce.

👉 A soft bounce happens when the email reaches the recipient’s mail server but is returned for temporary reasons.

This could be because their inbox is full, the server is down, or the message is too large. Think of it as knocking on a door when no one’s home. You might try again later and get through.

👉 On the other hand, a hard bounce is more serious. It means your email can’t be delivered due to a permanent issue.

This could be because the email account doesn’t exist, the domain name is wrong, or the recipient’s mail server has blocked delivery. It’s like sending a letter to an address that doesn’t exist. No matter how many times you try, it won’t get there.

Understanding the difference between these two types of bounces is key. A soft bounce might resolve itself, but a hard bounce needs action on your part.

Email bounce codes for soft bounces

Source

Below, you’ll find a list of the most popular email bounce codes for soft bounces and their meanings:

• 4.0.0 – Temporary failure: a general temporary failure without specific details.
• 4.1.0 – Other address status: an unspecified address-related issue that is temporary.
• 4.1.1 – Bad destination mailbox address: the destination mailbox address is invalid or does not exist.
• 4.1.2 – Bad destination system address: the system specified in the destination address is invalid.
• 4.1.3 – Bad destination mailbox address syntax: the syntax of the destination mailbox address is invalid.
• 4.1.4 – Destination mailbox address ambiguous: the destination address is ambiguous.
• 4.1.5 – Destination mailbox address valid: the destination address is valid but the mailbox is not accepting messages.
• 4.1.6 – Mailbox has moved: the mailbox has moved and is temporarily unavailable.
• 4.1.7 – Bad sender’s mailbox address syntax: the syntax of the sender’s mailbox address is invalid.
• 4.1.8 – Bad sender’s system address: the system specified in the sender’s address is invalid.
• 4.2.0 – Mailbox full: the recipient’s mailbox is full and cannot accept more messages.
• 4.2.1 – Mailbox disabled, not accepting messages: The mailbox is disabled and not accepting messages.
• 4.2.2 – Mailbox not accepting messages: the mailbox is not currently accepting messages.
• 4.3.0 – Mail system storage full: the mail system is full and cannot store any more messages.
• 4.3.1 – Mail system full: insufficient system storage.
• 4.3.2 – System not accepting network messages: the system is not accepting network messages.
• 4.3.3 – System not capable of selected features: the system is not capable of accepting the message with the selected features.
• 4.4.0 – Other network or routing status: unspecified network or routing issue.
• 4.4.1 – No answer from host: no response from the recipient’s email server.
• 4.4.2 – Bad connection: poor connection quality.
• 4.4.3 – Routing server failure: the routing server failed to route the message.
• 4.4.4 – Unable to route: the system is unable to route the message.
• 4.4.5 – Mail system congestion: the mail system is congested and cannot process the message.
• 4.4.6 – Routing loop detected: a routing loop was detected.
• 4.4.7 – Delivery time expired: the message could not be delivered within the time limit.
• 4.5.0 – Other protocol status: unspecified protocol issue.
• 4.5.1 – Invalid command: the command was not recognized by the recipient’s server.
• 4.5.2 – Syntax error: there was a syntax error in the command.
• 4.5.3 – Too many recipients: the message was sent to too many recipients.
• 4.5.4 – Invalid command arguments: the command arguments are invalid.
• 4.5.5 – Wrong protocol version: the protocol version is incorrect.
• 4.6.0 – Other or undefined media error: unspecified media error.
• 4.6.1 – Media not supported: the media type is not supported.
• 4.6.2 – Conversion required and prohibited: a required media conversion is prohibited.
• 4.6.3 – Conversion required but not supported: the required media conversion is not supported.
• 4.6.4 – Conversion with loss performed: the media was converted but with some data loss.
• 4.6.5 – Conversion failed: the media conversion failed.
• 4.7.0 – Other or undefined security status: unspecified security issue.
• 4.7.1 – Delivery not authorized, message refused: the delivery was not authorized and the message was refused.
• 4.7.2 – Mailing list expansion prohibited: mailing list expansion is prohibited.
• 4.7.25 – Anti-spam filter triggered: the email was temporarily blocked due to a spam filter.

These soft bounce codes signal temporary issues that might resolve themselves or can be addressed with a retry. They help diagnose and troubleshoot email delivery problems effectively.

Examples:

4.0.0:

4.2.2:

Email bounce codes for hard bounces

Here is a comprehensive list of email bounce codes for hard bounces, along with explanations:

• 5.0.0 – Permanent failure: a general permanent failure with no specific details.
• 5.1.0 – Other address status: an unspecified permanent failure related to the address.
• 5.1.1 – Bad destination mailbox address: the recipient’s email address does not exist.
• 5.1.2 – Bad destination system address: the destination system specified in the email address does not exist or is incorrect.
• 5.1.3 – Bad destination mailbox address syntax: the syntax of the recipient’s email address is invalid.
• 5.1.4 – Destination mailbox address ambiguous: the recipient’s email address is ambiguous.
• 5.1.5 – Destination mailbox address valid: the recipient’s email address is valid but the mailbox is not accepting messages.
• 5.1.6 – Mailbox has moved: the recipient’s mailbox has moved and no forwarding address is available.
• 5.1.7 – Bad sender’s mailbox address syntax: the syntax of the sender’s mailbox address is invalid.
• 5.1.8 – Bad sender’s system address: the system specified in the sender’s address is invalid.
• 5.2.0 – Mailbox disabled, not accepting messages: the recipient’s mailbox is disabled and not accepting messages.
• 5.2.1 – Mailbox full: the recipient’s mailbox is full and cannot accept any more messages.
• 5.2.2 – Mailbox not accepting messages: the mailbox is not currently accepting messages.
• 5.2.3 – Message too large: the message exceeds the recipient’s email server size limit.
• 5.2.4 – Mailing list expansion issue: an issue occurred when expanding a mailing list.
• 5.3.0 – Mail system full: the mail system storage is full.
• 5.3.1 – Mail system storage full: insufficient system storage.
• 5.3.2 – System not accepting network messages: the system is not accepting network messages.
• 5.3.3 – System not capable of selected features: the system is not capable of accepting the message with the selected features.
• 5.3.4 – Message too big for the system: the message size exceeds the system’s capabilities.
• 5.4.0 – Other network or routing status: unspecified network or routing issue.
• 5.4.1 – No answer from the host: no response from the recipient’s email server.
• 5.4.2 – Bad connection: poor connection quality.
• 5.4.3 – Routing server failure: the routing server failed to route the message.
• 5.4.4 – Unable to route: the system is unable to route the message.
• 5.4.5 – Mail system congestion: the mail system is congested and cannot process the message.
• 5.4.6 – Routing loop detected: a routing loop was detected.
• 5.4.7 – Delivery time expired: the message could not be delivered within the time limit.
• 5.5.0 – Other protocol status: unspecified protocol issue.
• 5.5.1 – Invalid command: the command was not recognized by the recipient’s server.
• 5.5.2 – Syntax error: there was a syntax error in the command.
• 5.5.3 – Too many recipients: the message was sent to too many recipients.
• 5.5.4 – Invalid command arguments: the command arguments are invalid.
• 5.5.5 – Wrong protocol version: the protocol version is incorrect.
• 5.6.0 – Other or undefined media error: unspecified media error.
• 5.6.1 – Media not supported: the media type is not supported.
• 5.6.2 – Conversion required and prohibited: a required media conversion is prohibited.
• 5.6.3 – Conversion required but not supported: the required media conversion is not supported.
• 5.6.4 – Conversion with loss performed: the media was converted but with some data loss.
• 5.6.5 – Conversion failed: the media conversion failed.
• 5.7.0 – Other or undefined security status: unspecified security issue.
• 5.7.1 – Delivery not authorized, message refused: The delivery was not authorized and the message was refused.
• 5.7.2 – Mailing list expansion prohibited: mailing list expansion is prohibited.
• 5.7.3 – Security conversion required but not possible: a required security conversion is not possible.
• 5.7.4 – Security features not supported: the security features required are not supported.
• 5.7.5 – Cryptographic failure: a cryptographic failure occurred.
• 5.7.6 – Cryptographic algorithm not supported: the cryptographic algorithm used is not supported.
• 5.7.7 – Message integrity failure: the message integrity check failed.

These hard bounce codes indicate permanent issues that generally require action to resolve, such as correcting the recipient’s email address or resolving security issues.

Examples:

5.0.0:

5.1.1:

5.5.0:

How to prevent email bounces: top 10 ways

To stay away from ever seeing such codes again, it’s important that you do everything to not let your emails bounce.

Below, we share top strategies to make your emails reach their recipients:

Validate your email addresses before sending out emails

Before you hit send, always clean and validate your email list.

This can be done using awesome tools like Bouncer.

Bouncer helps you check if the email addresses are valid without sending an actual email.

It verifies the syntax, checks DNS and MX records, and even uses smart algorithms to connect with the recipient’s email server.

This way, you avoid traditional bounce codes and those annoying network messages.

👉 See how it works today .

Use double opt-in

Getting your subscribers to confirm their email addresses with a double opt-in process is a smart move.

This not only confirms that the email is valid but also that the recipient really wants to hear from you.

It’s a great way to keep your list clean and full of engaged contacts.

Segment your email lists

Not all your subscribers are the same, so don’t treat them that way.

Segment your email lists to send more personalized and relevant content.

This boosts engagement and reduces the chances of your emails bouncing.

Send engaging content

Boring or irrelevant content can lead to poor engagement.

When your emails aren’t opened or clicked, the recipient’s email server might start treating them as spam.

Keep your content interesting and valuable to your audience.

Monitor your email deliverability

Bouncer’s Deliverability Kit is your go-to tool for this.

It helps you test your inbox placement, verify your authentication, and monitor blocklists.

Knowing your enhanced SMTP bounce codes and monitoring blocklists can save you from a lot of trouble.

Maintain a consistent sending schedule

Sending emails erratically can confuse the recipient’s email server and lead to bounces.

Stick to a consistent schedule so your subscribers know when to expect your emails.

Choose a reputable email service provider

Using a well-known email service provider (ESP) improves your deliverability.

They have established relationships with mail server administrators and can handle retries with the recipient mail server more effectively.

Use a recognizable sender name and email address

Make your sender name and email address look familiar to your recipients.

This helps them recognize your emails and lowers the chances of being marked as spam.

Watch out for enhanced bounce codes

Pay attention to enhanced bounce codes which give more detailed information about why an email bounced.

Issues like insufficient system storage or a bad sender’s system address can be identified and fixed.

Keep an eye on message integrity

Sometimes, your email might get rejected due to message integrity failure.

Make sure your emails are correctly formatted and authenticated to avoid these issues.

By following these tips, you’ll minimize email bounces and your messages will reach your recipients’ inboxes.

Conclusion

Understanding email bounce codes can be a game-changer for any email marketer.

By decoding these messages, you can pinpoint the issues and take steps to fix them, all so your emails reach their intended recipients.

From handling soft bounces caused by temporary issues to addressing hard bounces that signal permanent problems, being equipped with this knowledge puts you in control.

So, the next time you see a bounce notification, you’ll know exactly what to do. Keep your campaigns on track, and turn those bounces into successful deliveries.

P.S. Remember to use Bouncer if you need top-level assistance with avoiding email bounces.