When an email is not delivered to the intended recipient(s), it’s most commonly known as a ‘bounce’. When an email bounces, the receiving email server sends a notification, known as Non-Delivery Report (NDR), back to the sender to inform them that the email has not been delivered. The notification will include important information to help you identify the reason for the email delivery failure such as time and date the message bounced; the mail server that bounced it and the RFC code* with the reason for the bounce. A bounce usually falls into two categories: hard and soft bounces.

Hard Bounces

A hard bounce is an email which is considered to be ‘permanently unavailable’. They are often caused by sending emails to unknown or non-existent domains which more often than not results from questionable data sources or spam and it can be a cause for an email address to be blacklisted. Most commonly hard bounces happen when:

  • You’re sending to inactive or non-existent email addresses;
  • You’re sending to non-existent domain names; or
  • There’s a blocked delivery by the recipient email server.

Soft Bounces

A soft bounce is an email which is considered to be ‘temporarily unavailable’. They often occur when the server rejects the email due to temporary conditions. Usually, there are three to five attempts made sending emails considered as soft bounces before they turn into hard bounces. The most common reasons soft bounces occur are:

  • Recipient’s mailbox is full
  • Temporary domain failure; or
  • An account being disabled or inactive.

Nonetheless, soft bounces can also be the result of block bounces where the email server is rejecting the email due to filtering issues. Such as:

  • Previous complaints from a user;
  • Sender’s domain or IP address are found on a blacklist;
  • Content spam score is too high;
  • Problems with the authentication of the sender and/or sending source; or
  • Blocked URL.

Reducing Bounces

The best way to reduce the number of bounces is by following some key email deliverability practices such as:

  1. Maintain clean email list: check your lists regularly in order to ensure there are no invalid emails or non-responders;
  2. Use double opt-in: by sending a confirmation email to users subscribing to your email list(s) you ensure that said user's email addresses are not only valid but they, in fact, want to receive your emails;
  3. Monitor email delivery: track your email delivery rates by paying attention to not only the bounce rates but the response rates as well. When you regularly monitor the deliverability, you can notice potential failures even before they happen or before they do too much damage.

Where can I find the bounce rates?

Your bounce rates can be found in the Reporting section (left-hand-side menu).

From there navigate to the email campaign for which you’d like to see the reporting for and then go to ‘Fails’.

You’ll be presented with data for the number of emails sent, the bounces, the unsubscribed contacts and the complaints. In order to drill into the bounce rate report click on ‘Bounces’.

Once within the report, you will the recipient’s email address, the suppression status (active or suppressed), the bounce type (soft or hard) and the diagnostics.

You can also download the reporting in different formats - Excel, PDF and CSV.

There are a lot of soft and hard bounce error codes (depending on the email providers) which might not be listed in this article. If you come across any not listed here, please either use the chat box at the bottom right corner or email us at support@emailout.com and we’ll be more than happy to take a further look.

*According to the RFC, hard bounces are depicted by a 5XX code and soft bounces by a 4XX code. However, keep in mind not all ISPs adhere to that code so there could be exceptions to the rule.

Bounce Error Codes and Fixes

Soft Bounce Error Codes:

4.4.1 - Network Is Busy
Reason:
The email has not been delivered because the recipient’s email provider didn’t respond to the sender’s email provider’s request to connect.

Fix: The sender’s system will retry sending the message a few times and give further details.  If the sender is not notified within a few days that the message has been delivered attempt to send it again.

4.2.2 - Mailbox Is Full
Reason
: The email hasn’t been delivered because the recipient’s email inbox is full.

Fix: The sender’s email system will, probably, attempt sending the message automatically and send you updates. Nonetheless, you may also want to contact the recipient (phone, text message, etc.) and ask them if they are aware their inbox is full.

4.4.2 - Network Connection Timed Out
Reason
: The email hasn’t been delivered because the sender’s and recipient’s email providers got disconnected before the email had been handed over for delivery.

Fix: The sender’s system will retry sending the message a few times and give further details. If the sender is not notified within a few days that the message has been delivered attempt to send it again.

4.4.4 - Email Routing Issue Due To Network
Reason
: The email hasn’t been delivered due to network problems with the recipient’s email provider limit routing the message.

Fix: Wait a while as the receiving email server is busy. You may also want to double check the recipient’s email address when sending the message again. If the error continues, consider contacting the recipient through alternative email addresses (or other means).

5.0.0 - Unknown Issue
Reason
: The email couldn’t be delivered due to an unknown issue.

Fix: Check the recipient’s email address for any misspellings or missing letters (symbols). If there is an error correct it and try again. Make sure you've also corrected the issue is your contact list, too. If the problem continues try contacting the recipients another way (phone, text message, etc.) to confirm their email address.

5.2.0 - Unknown Issue With Mailbox
Reason
: The email couldn’t be delivered due to an unidentified problem with the recipient’s email inbox.

Fix: Check the recipient’s email address for any misspellings or missing letters (symbols). If there is an error correct it and try again. Make sure you've also corrected the issue is your contact list, too. If the problem continues try contacting the recipients another way (phone, text message, etc.) to confirm their email address.

5.2.1 - Mailbox Is Not Accepting New Messages
Reason
: The email couldn’t be delivered because the recipient’s email inbox is currently not accepting new messages.

Fix: Contact the recipient another way (phone, text message, etc.) in order to check when their email will be running again. Alternatively, attempt to send the message again in a few hours, or the next day.

5.2.2 - Mailbox Is Full
Reason
: The email couldn’t be delivered because the recipient’s email inbox is full.

Fix: Contact the recipient another way (contact, text message, etc.) and ask them if they are aware their inbox is full. The recipient can delete some emails to get available space.

5.3.1 - Email System Is Full
Reason
: The email couldn’t be delivered because the recipient’s email system has run out of space and can’t store any more messages.

Fix: Wait a few minutes, double check the recipient’s email address and attempt to send again. If the error continues, try contacting the recipient in an alternative way.

5.4.5 - Overloaded Network
Reason
: The email couldn’t be delivered due to the recipient's network being congested or overloaded.

Fix: Wait a bit considering the email server is busy. The sender’s email server will retry sending the message a few time to see if it gets through and provide further details. If the error continues, try contacting the recipient in an alternative way.

5.4.14 - Hop Count Exceeded – Possible Mail Loop
Reason
: The email couldn’t be delivered because it exceeded the hop count and/or a routing loop cause the email message to be forwarded too many times, either because of incorrect routing tables or a user-forwarding loop.

Fix: Double check the recipient’s email address to make sure it’s correct and attempt to send the message again. If the error occurs again, contact your system administrator to check the configuration settings. In the meantime, contact the recipient via an alternative email address or other methods.

5.5.3 - Too Many Recipients For Message
Reason
: The email couldn’t be delivered because it was addressed to too many recipients and the email system involved couldn’t handle it.

Fix: Try dividing the list of recipients into two groups and send the email to each group separately. If the problem still occurs, keep diving the recipient’s list further.

5.7.1 - Message Sender Blocked By Receiving Server
Reason
: The email couldn’t be delivered because the recipient’s email provider is not accepting messages from this sender. Usually, this means the receiving system has security or policy concerns with regards to either the specific sender, recipient or the sender’s email system.

Fix: Attempt sending the message again to see if it was a one-time problem or if the same error occurs again. If it does, contact the sender’s email support team as the email system may have a security issue.

Hard Bounce Error Codes:

5.1.0 - Unknown Issue With Email Address
Reason
: The email couldn’t be delivered because there is some sort of a problem with the recipient’s email address.

Fix: Check the recipient’s email address for any misspellings or missing letters. If there is an error correct it. If the error occurs again, contact the recipient via alternative methods to confirm the email address.

5.1.1 - User Name Of Email Address Not Valid
Reason
: The email couldn’t be delivered because the mailbox for the recipient’s email address is either invalid or it does not exist.

Fix: Check the local-part (username before the ‘@’) of the recipient’s email address for misspellings or missing letters. If an error is found, correct it in the contact list. If the email address looks correct, there might be a problem with the recipient’s domain. If necessary, contact the recipient via alternative methods to confirm their email address. You might also want to contact your email provider for help.

5.1.2 - Domain Name Of Email Address Not Valid
Reason
: The email couldn’t be delivered because the recipient’s email provider states that the recipient’s email address does not exist or cannot receive emails.

Fix: Check the recipient’s email address for any misspellings or missing letters. If an error is found, correct it in the contact list. It is also possible that the domain is temporarily inactive. If the spelling looks correct, contact the provider.

5.1.3 - Email Address Malformed
Reason
: The email couldn’t be delivered because the recipient’s email address is not a valid email address.

Fix: Check the recipient’s email address for spaces or unusual characters. If an error is found, correct it in the contact list. If necessary, contacts the recipient via alternative methods to confirm their email address.

5.1.4 - Multiple Mailboxes For Recipient
Reason
: The email couldn’t be delivered because the recipient’s email address points to more than one email inbox and the recipient’s email provider doesn’t know which one to use.

Fix: Check the recipient’s email address for misspellings or missing letters. If there are any, correct them in the contact list. If necessary, contact the recipient via alternative methods to confirm their email address.

5.1.6 - Email Address No Longer Accepts Or Forwards Messages
Reason
: The email couldn’t be delivered because the recipient’s email address no longer accepts email messages (even though it did in the past). 

Fix: Check the recipient’s email address for misspellings or missing letters. If there are any, correct them in the contact list. If necessary, contact the recipient via alternative methods to confirm their email address.

5.1.7 - Sender's Address Is Written Incorrectly
Reason
: The email couldn’t be delivered because the sending address is not a valid email address.

Fix: Review the sender’s email address in order to identify any misspellings or missing letters. In the settings or options of the sender’s email system (program) or webmail service, ensure that your own email address is entered correctly.

5.1.8 - Sending Address Is Bad Or Can't Accept Replies
Reason
: The email couldn’t be delivered because the sender’s email address either does not exist or cannot accept return emails.

Fix: Review the sender’s email address in order to identify any misspellings or missing letters. In the settings or options of the sender’s email system (program) or webmail service, ensure that your own email address is entered correctly. If the problem persists, contact the sender’s email support team.

5.2.3 - Message Is Too Long
Reason
: The email couldn’t be delivered because it was longer than the recipient’s email system allows.

Fix: Check that the message contacts only the required text and attachments (some systems count attachments as part of the message length). You can also try to divide, if possible, the text and/or attachments into two or more separate emails. If the issue persists even when shortening the message and/or removing the attachments, contact the recipients another way and suggest they contact their email provider for help.

5.2.4 - Mailbox Has A Mailing List System Issue
Reason
: The email couldn’t be delivered because the recipient’s email address, which is a mailing list, had trouble forwarding the message.

Fix: Check that the recipient’s email address is the correct one for the mailing list you want to reach. If there’s an error in the email address, correct it. You may also consider contacting the mailing list administrator for support.

5.3.0 - Unknown Issue With The Receiving System
Reason
: The email couldn’t be delivered due to an unknown problem with one or more of the recipient’s email systems.

Fix: Double check that the recipient’s email address is correct. Wait a few minutes (hours) then try sending the message again. If necessary, contact the recipient via alternative methods to confirm their email address.

5.3.2 - Receiving Email System Not Operable
Reason
: The email couldn’t be delivered because one or more of the recipient’s email systems is not accepting messages, possible due to an unplanned shutdown, excessive overload or system maintenance.

Fix: Attempt sending the message later. If the issue still persists, contact the recipient via alternative methods to confirm their email address.

5.3.3 - Message Features Not Supported
Reason
: The email couldn’t be delivered because the message you’re attempting to send seems to contain features which are not supported by the receiving system.

Fix: Contact the sender’s email system support team to investigate what features of your email system may not be supported by the receiving system. Additionally, consider contacting the recipient via alternative methods.

5.3.4 - Message Is Too Large
Reason
: The email couldn’t be delivered because the recipient’s email provider has a limit on the message’s size and the email you’re trying to send is exceeding it.

Fix: Check the body of the message for large amounts of text or for any large attachments. Try adjusting the content or reducing the attachment size in order to fit the requirements and try resending it. If the issue continues, you might have to thoroughly review and edit the content (and attachments). 

5.4.0 - Unknown Issue With Network Connection
Reason
: The email couldn’t be delivered because of an unknown network connection problem between the email providers.

Fix: Check for any updates regarding connectivity issues with both providers. They might be working on resolving them so give them some time and then try resending.

5.4.1 - Network Is Busy
Reason
: The email couldn’t be delivered because the recipient’s email provider didn’t respond to the sender’s email provider request to connect.

Fix: The sender’s email provider may try re-sending the message a few more times and provide a further update. If the sender is not notified within a few days that message was delivered, try sending it again. 

5.4.2 - Network Connection Timed Out
Reason
: The email couldn’t be delivered because the sender’s and the recipient’s email providers got disconnected before the message had been handed over for delivery.

Fix: The sender’s email program will attempt resending the message a few more times and provide further details. If the sender is not notified within a few days that the message has been delivered try re-sending it.

5.4.3 - Network Issue With The Receiving System
Reason
: The email couldn’t be delivered because a network problem prevented the recipient’s email provider from delivering the message.

Fix:  The sender’s email program will attempt resending the message a few more times and provide further details. If the sender is not notified within a few days that the message has been delivered try re-sending it.

5.4.4 - Email Routing Issue Due To Network
Reason
: The email couldn’t be delivered due to network issues with the recipient’s email provider limit routing the message.

Fix: The sender’s email provider is certainly working on resolving the issue. In the meantime, check the recipient’s email address for any possible misspellings and consider contacting them via alternative methods to confirm their email address. 

5.4.6 - Network Issue Caused A Mail Loop
Reason
: The email couldn’t be delivered because there was a network issue causing a routing loop retrospectively causing the message to be forwarded over and over.

Fix: Double check the recipient’s email address to ensure it’s correct and try resending the message. If that doesn’t work, consider contacting the recipient via alternative email or other means.

5.4.7 - Message Expired Before Delivery Completed
Reason
: The email couldn’t be delivered because the message was considered too old by the receiving system. It may have been delayed while being processed.

Fix: Double check the recipient’s email address to ensure it correct and try resending the message. If that doesn’t work, consider contacting the recipient via alternative email or other means.

5.5.0 - Unknown Issue With Your Email Server
Reason
: The email couldn’t be delivered because the sender’s email server had an unknown technical issue and couldn't relay the message properly.

Fix: Double check the recipient’s email address to ensure it’s correct and try resending the message. If it doesn’t work, consider contacting the recipient via an alternative email address or other means.

5.5.1 - Bad Messaging Command In Transmission
Reason
: The email couldn’t be delivered because the sender’s email provider used a messaging command that was not supported.

Fix: Attempt resending the message to see if it was a one-time issue or if the same error will occur again. If the issue persists, contact the support team as the sender’s email system issued a command not supported by the recipient’s email provider.

5.5.2 - Email System Sent Bad Command
Reason
: The email couldn’t be delivered because the sender’s email provider email system used a messaging command that the recipient’s email provider didn’t recognise.

Fix: Attempt resending the email message see if the issue was a one-time thing or if the same error will occur again. If the issue persists, contact the support team as the sender’s email system issued a command not supported by the recipient’s email provider.

5.5.4 - Email System Sent Bad Command
Reason
: The email couldn’t be delivered because the sender’s email provider email system used a messaging command that the recipient’s email provider didn’t recognise.

Fix: Attempt resending the email message see if the issue was a one-time thing or if the same error will occur again. If the issue persists, contact the support team as the sender’s email system issued a command not supported by the recipient’s email provider.

5.5.5 - Email Systems Were Incompatible
Reason
: The email couldn’t be delivered because the sender’s and the recipient’s email programs are using incompatible versions of the software.

Fix: Try resending the message again in a bit, since the issue might be temporary, or already in the process of being resolved. If the problem persists, contact the respective email providers for help.

5.6.0 - Unknown Issue With The Message Content
Reason
: The email couldn’t be delivered due to an unknown issue related to the message’s content or the attachment(s).

Fix: If your message has attachments, try resending it without them and then email each attachment separately. If the error occurred where there were no attachments, check the text of the message for unusual characters and/or content. If the error continues, contact the email provider for assistance.

5.6.1 - Message Content Or Attachment Not Supported
Reason
: The email couldn’t be delivered because it included content and/or attachments that the recipient’s email provider couldn’t accept.

Fix: If the message has attachments try sending each one in a separate email. Often, things like Calendar Invitations or Meeting Reminders can have a format which causes issues. If that’s the case see if you can make an adjustment and resend the message. If the issue is content related, review your message’s text again and if necessary make edits and resend the email. If the issue persists, contact the email provider for help.

5.6.2 - Message Content Not Accepted By Recipient Server
Reason
: The email couldn’t be delivered because it included content and/or attachments that needed to be converted and the conversion was not allowed.

Fix: If the message had attachment try sending it again without them and then send each one in a separate email. If the issue occurred when there were no attachments, check the message’s text for unusual content and/or characters. If the error persists, contact the email provider.

5.6.3 - Message Content Could Not Be Processed For Delivery
Reason
: The email couldn’t be delivered because it included content and/or attachments that needed to be converted and the email system was unable to accomplish the conversion.

Fix: If the message had attachment try sending it again without them and then send each one in a separate email. If the issue occurred when there were no attachments, check the message’s text for unusual content and/or characters. If the error persists, contact the email provider.

5.6.4 - Content Loss During Delivery
Reason
: The email couldn’t be delivered because it included content and/or attachments that needed to be converted and the information was lost in the process.

Fix: First, contact the recipient(s) to ensure that no undesirable consequences result from the information being lost from this message before it was delivered. Second, if the message had attachments try to resend it without them and then send each attachment separately. If the error occurred when there were no attachments, check the message’s text for unusual content and/or characters. If the issue occurred only with a specific type of attachment, consider sending the information in a different format (if possible).

5.6.5 - Message Content Not Accessible To Recipient Server
Reason
: The email couldn’t be delivered because it included content and/or attachments that needed to be converted but the conversion was not completed.

Fix: If the message had attachment try sending it again without them and then send each one in a separate email. If the issue occurred when there were no attachments, check the message’s text for unusual content and/or characters. If the error persists, contact the email provider.

5.7.0 - Unknown Security Or Policy Concern
Reason
: The email couldn’t be delivered because the recipient’s email system is not accepting messages from this sender. Usually, this means the receiving system has flagged either the individual sender or the sending mail servers for security concerns or for suspicion of spam-related activity.

Fix: Try resending the message to check if it was a one-time issue or if the same error will occur again. If the same error occurs, contact your email support team as either your computer or your email system may have a security issue.

5.7.2 - Not Authorised To Send To Mailing List
Reason
: The email couldn’t be delivered because 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.

Fix: If it seems like your email address has been blocked unintentionally, try resending the message in a while in case the issue was temporary. If the error occurs again, contact the mailing list administrator.

5.7.3 - Security Features Could Not Be Interpreted
Reason
: The email couldn’t be delivered because the recipient’s and the sender’s email programs had issues with the security protocols between them.

Fix: Attempt sending the message again to check if it was a one-time issue or if the error will occur again. If the issue persists, contact the support teams as the email systems may have a security issue.

5.7.4 - User Authentication For Message Failed
Reason
: The email couldn’t be delivered because there was a problem with the sender’s user authentication to the sender’s email provider email system.

Fix: Review the email account settings. Check for options that require ‘user authentication’ for outbound SMTP and ensure that the username and password values are correct. If the account settings are correct, contact the email provider to help identify any misconfigurations with the sender’s account or with the email system.  

5.7.5 - Message Authentication Issues
Reason
: The email couldn’t be delivered because the sending email system uses DKIM, an authentication mechanism. Either the sending or the receiving email system had some issues with the sending system’s DKIM.

Fix: Try resending the email message to check if the issue was a one-time one or if the error will occur again. If the problem continues, contact the sender’s email support team to help identify misconfigurations with the sender’s account or with the email system. In particular, issues with the use of email authentication such as DKIM should be investigated. 

5.7.6 - Message Encryption Issues
Reason
: The email couldn’t be delivered because an email server involved with sending the message had difficulty processing the security features of the message.

Fix: If you’re intentionally sending encrypted email messages, check the sender’s email program settings and (if required) make the necessary changes. Resend the message to see if the same error will occur again or if it was a one-time issue. If the problem continues, contact the sender’s email support team or encryption provider to help identify any misconfigurations with the sender’s account or encryption tool.

5.7.7 - Potentially Corrupt Message
Reason
: The email couldn’t be delivered because an email server involved with sending the message had difficulty processing the security features of the email message. The message was probably corrupt or altered in some way during sending.

Fix: If you’re intentionally sending encrypted email messages, check the sender’s email program settings and (if required) make the necessary changes. Resend the message to see if the same error will occur again or if it was a one-time issue. If the problem continues, contact the sender’s email support team or encryption provider to help identify any misconfigurations with the sender’s account or encryption tool.

9.1.1 - Hard bounces with no bounce code found
Reason
: The email couldn’t be delivered because either the email address was invalid or the message was rejected by the mail server (such as from a sending limit).

Fix: Double check the recipient’s email address to ensure it’s correct and try resending the message. It might’ve been a one-time issue. If the error persists, contact the recipient to check if their email address is correct. In case the issue still continues, contact the mail service provider for help.

Bounces, especially hard ones, are damaging the sender reputation. If you have a high bounce rate, it means you probably have a lot of non-existent email addresses in your database. By sending to non-existent email addresses you can hit a spamtrap. In order to avoid harming your sender reputation, we recommend reducing your bounces.

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