What is a bounce and how can I prevent it?

This article explains what a bounce is, how a bounce occurs, what the consequences of a bounce are, and how to respond to a bounce.

What is a bounce?

A bounce indicates that the email you wish to send to a contact cannot be delivered or received. If that's the case, the email essentially 'bounces back', hence the name 'bounce'. This shows that an email has been rejected and therefore did not land in the recipient's inbox.

How does a bounce occur?

The occurrence of a bounce can have many different causes, distinguishing between a 'soft bounce' and a 'hard bounce'. Below you will find an overview of the most common reasons for a bounce.

  • The email address does not exist (hard bounce)
    These types of bounces have a bounce code starting with 5xx. It is normal to have a handful of contacts with this status after sending out an email, as email providers shut down accounts that have not been used for years. With a hard bounce, a contact in MailBlue will be marked as 'rejected'. With this status, the contact will no longer be able to receive emails.
  • Temporary bounce (soft bounce)
    These types of bounces have a code starting with 4xx or 5xx. There are many different temporary errors that can occur, such as a full inbox or a temporary issue with the inbox provider's server preventing an email from being received. If the same contact experiences three temporary bounces in a row with three different outgoing emails, this contact will be marked as 'rejected' in MailBlue, similar to a hard bounce. In the case of a soft bounce, the system will not attempt to redeliver the same email to a contact, unless a specific type of bounce is received with the error 'try again later'.
  • Sender reputation
    If you receive many spam reports because recipients mark your email as 'spam' instead of unsubscribing from the mailing list, certain email providers may block messages from your email address based on a poor sending reputation.
  • Content of your message
    Sometimes the content of your message can cause the email to be blocked by receiving email providers. This may occur if your message contains a spam-sensitive phrase, a poor image, or a link to a malware website, for example. MailBlue checks the content of your email and displays the results in the internal spam check, in the summary of the respective email.
  • DMARC record with restrictions
    This can cause legitimate messages to bounce. You can use a tool like this one to check your DMARC record. If you have a 'p=reject' record, this will result in your messages bouncing if you have not set up DKIM.
  • The 'From' sender email address
    If you send from a free sender email address - such as @yahoo.com, @gmail.com, or @hotmail.com - emails will not be sent/received, resulting in a bounce.
  • 'Auto-bounce' bad email address
    The system clearly bounces bad email addresses, such as 12345678910@12345.com. These contacts will be shown in the report as a bounce with a 9.1.5 bounce code. In many cases, these are spam contacts present in your account.


How can I find all bounced email addresses in MailBlue?


In your 'Contacts', you can filter by the status of contacts to find out which contacts have bounced for at least one list. You can do this by filtering on the 'Rejected' status.


Where can I see the reason for a bounce?

When you see that a contact has the 'rejected' status, you can look up the bounce code to determine the cause. You can find the bounce code of a contact in the 'Reports' section within your account. Here you can view which contacts have bounced per campaign and which bounce code is associated with it. To do this, navigate to 'Campaigns' and select a campaign of your choice, then click on 'Bounces' under the 'Summary' tab to see the bounces of that specific email. You will find the bounce code under the 'Code' heading.
The bounce codes displayed in reports are the codes our servers receive from the recipients' mail servers. Be aware that sometimes the codes we receive from these email servers may contain codes that do not always indicate the actual reason for the bounce.

Below you will find a list of bounce codes that you may encounter in MailBlue. Based on the code, you will see what the cause of the bounce is.

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Bounce Code Bounce Type Description
4.2.2 Soft The mailbox is full because the recipient has exceeded an administrative quota or physical capacity per mailbox.
5.0.0 Hard The email address does not exist.
5.1.0 Hard Your email ID is blocked by the recipient.
5.1.1 Hard The left part of the email address (before the '@') is invalid.
5.1.2 Hard The domain, the right part of the email address (after the '@') is invalid.
5.1.3 Hard The complete email address is invalid.
5.1.4 Hard The receiving email address points to more than one email inbox and the recipient's email provider does not know which one to use.
5.1.5 Hard The email cannot be delivered even though the receiving email address is valid. This bounce will not be resolved by resending the email in its current form. Adjustments to the message are necessary to ensure successful delivery of the email.
5.1.6 Hard The receiving email address no longer accepts emails (in general), even though it used to do so in the past.
5.1.7 Hard The sender's address is invalid. This can apply to any part of the email address.
5.1.8 Hard The sender's email system does not exist or cannot accept bounce-back emails. For domain names, this means that the part of the address to the right of the '@' is invalid for email.
5.2.0 Soft There is an issue with the recipient's email inbox. The mailbox exists, but something unidentifiable in the receiving mailbox caused this bounce notification.
5.2.1 Soft The mailbox exists but does not accept messages. This can be a permanent error if the mailbox is never re-enabled or a temporary error if the mailbox is only temporarily disabled.
5.2.2 Soft The mailbox is full because the recipient has exceeded an administrative quota or physical capacity per mailbox.
5.2.3 Hard

The message is longer than what the recipient's email system allows.

5.2.4 Hard

The mailbox is experiencing an issue with the mailing list system. The message cannot be delivered because the recipient's email address, which is part of a mailing list, is having trouble forwarding the email to the list members.

5.3.0 Hard

There is an unknown issue with one or more recipient email systems.

5.3.1 Soft

The message cannot be delivered because the recipient's email system is out of space and cannot store more messages. The recipient may be unable to delete material to make room for additional messages.

5.3.2 Hard

One or more recipient email systems are not accepting messages, possibly due to an unplanned shutdown, excessive load, or system maintenance.

5.3.3 Hard

The message you are sending contains features not supported by the receiving system. This usually happens when systems have different capabilities.

5.3.4 Hard

The recipient's email provider has a maximum message size, and the message you sent is larger.

5.4.0 Hard

There is an unknown issue with the network connection between the email providers.

5.4.1 Hard The recipient's email provider did not respond to the sender's request to establish a connection.
5.4.2 Hard The outgoing connection was established but could not complete the message transaction due to a timeout or insufficient connection quality.
5.4.3 Hard A network issue caused the recipient's email provider to be unable to deliver the message.
5.4.4 Hard

Network issues with the recipient's email provider are limiting the routing of this message.

5.4.5 Soft The email system was unable to deliver the message because the recipient's email network was congested or overloaded.
5.4.6 Hard There was a network issue causing a routing loop, resulting in this message being forwarded repeatedly.
5.4.7 Hard The message was considered too old by the receiving system. It may have been delayed during processing.
5.5.0 Hard The sender's email server had an unknown technical issue and could not relay the message properly.
5.5.1 Hard The sender's email server used a command that was not supported.
5.5.2 Hard

The sender's email provider used a command that the receiving email provider did not recognize.

5.5.3 Soft

The message was addressed to more recipients than the involved email systems could handle.

5.5.4 Hard

A valid mail transaction protocol command was issued with invalid arguments because the arguments were out of range or represented unrecognized functions.

5.5.5 Hard There was a mismatched protocol version that could not be automatically resolved by the communicating parties.
5.6.0 Hard Due to unknown reasons related to the content or attachments of the message, the message could not be delivered.
5.6.1 Hard

The content or attachments of the message could not be accepted by the recipient's email provider.

5.6.2 Hard

The message contained content or attachments that needed to be converted, which the email system is not allowed to do.

5.6.3 Hard The message contained content or attachments that needed to be converted, which the email system is unable to do.
5.6.4 Hard The message contained content or attachments that needed to be converted, resulting in information loss.
5.6.5 Hard The message contained content or attachments that needed to be converted but were not completed.
5.7.0 Hard The message caused a security issue, but it is not clear what exactly caused the problem.
5.7.1 Hard

The recipient's email provider does not accept messages from this sender. This usually means that the receiving system has security or policy issues related to the specific sender, recipient, or sender's email system.

5.7.2 Hard

The recipient's email provider has blocked the sender's email address from posting messages to the mailing list to which the message was addressed.

5.7.3 Hard The recipient's email program and the sender's email program had issues with their mutual security protocols.
5.7.4 Hard There was an issue with the sender's user authentication at the sender's email system.
5.7.5 Hard

The email sending system uses DKIM, an authentication mechanism. The sending or receiving email system had an issue with the DKIM of the sending system.

5.7.6 Hard The sender's email server had difficulty processing the security features of the message.
5.7.7 Hard

The sender's email server had trouble processing the security features of the message. The message may have been corrupted or altered in some way during transmission.

9.1.1 Hard Hard bounce without bounce code. This could be an invalid email address or a rejected email from your mail server (such as a sending limit).

What are the consequences of a bounce?

Bounces can have both a positive and negative impact on the deliverability of your emails. The frequency at which you experience bounces as a sender affects your sending reputation. When you experience a lot of bounces, receiving email providers are more likely to view your emails as 'risky' and therefore place them in the spam inbox. On the other hand, if you hardly experience any bounces, you are building a good sending reputation for your sender email address. A good sending reputation has a positive effect on the deliverability of your emails.The frequency at which you experience a bounce is also known as the bounce ratio. A low bounce ratio has positive effects on your deliverability. A bounce ratio of 0.5%-1% is quite normal.

How do I reduce my bounce ratio?

It may happen that your bounce ratio increases temporarily or is higher on a one-time basis, which can have various reasons. For example, you may have imported an old mailing list with many inactive contacts, resulting in a high number of bounces. It could also be due to an email triggering the spam filters of a receiving provider, or because a larger number of recipients have marked your email as spam.

Since the bounce ratio is closely related to your sending reputation and the deliverability of your emails, you naturally want to keep it as low as possible. Below are some tips to proactively address this.

  1. Ensure your DMARC settings are configured correctly, you can read more about this in this article.
  2. Use a double opt-in. Using a double opt-in has two advantages; firstly, it eliminates any invalid email addresses entered due to typos or spam subscriptions from robots. Secondly, you can be sure that the contacts confirming the double opt-in manage their email accounts and want to hear from you. If you use a MailBlue form to add contacts to a list, the double opt-in option will be 'ON' by default.
  3. Make sure to add a Captcha to your form, so that robots cannot sign up on the form and add spam contacts to your account, whether or not you have the double opt-in enabled. To learn more about adding a Captcha, read this article.
  4. Regularly send emails to your recipients. If you haven't sent any communication to your list for a long time, some email addresses may no longer exist. This can result in bounces, and the sender (MailBlue) may be marked as a spammer. Also, recipients may 'forget' about you if you don't email them regularly, risking that they mark your emails as 'spam' within their email provider.
  5. In MailBlue, there are two pre-built automation templates available that you can load by default, the Engagement Tagging automations. You can use these to manage the engagement of your contacts. This two-part automation will tag your contacts as 'Engaged', 'Disengaged', or 'Inactive', based on how much time has passed since they interacted with your communications. These tags can be used for analysis, segmentation, list cleaning, and triggering other automations. To learn more about these automations, click here.
  6. Use a real-time verification service as an additional security measure. Services like Briteverify can be used to identify if an email address is valid before submitting a form. This is recommended if you receive thousands of sign-ups per day.

How do I resolve a bounce?

A bounce occurs in two variants, namely a 'soft bounce' and a 'hard bounce'. Unlike a hard bounce, a soft bounce often resolves itself. In many cases, this is due to a temporary network issue or a full inbox on the recipient's side. When experiencing a soft bounce, you do not need to take immediate action. If you encounter three consecutive soft bounces with a single contact, it is advisable to contact this person to address the bounce together. It may be that a contact is unaware of their full inbox.

A hard bounce usually does not resolve itself, manual action is often required. The action to be taken depends on the bounce experienced by a contact. In this diagram you can find the bounce codes. By looking up the specific bounce code online, you will find additional information about this bounce and actions to resolve it.

If you experience an invalid bounce - for example, if it is indicated that an email address is not valid when it is - we ask you to send a message to our support department.

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