[101 Cheatsheat] Email Bounce Codes and How to Fix bounced or rejected emails?

Published on 2020-04-29ยท Updated on 2021-12-15


Sending out emails to large users are only helpful if your bounce rates are low. Sending emails with bounce rates more than 5% is a concern and anything around 10% is a serious concern to be addressed. If you're sending emails with higher bounce rates, then this can impact the reputation of your sending domain and IP address severely. It is recommended to weekly check the reputation of your sender domain and IP address to ensure your email practices comply with industry standards.

While launching an email campaign, you need to anyways segregate your email list into segments like Active and Inactive users. And, send emails to active users first. But, the active list is always around 10% of the total subscriber base. That's the hard truth, in most of the businesses so one cannot just rely on the active data to send emails.

So, what's the solution? How one can send emails to even an inactive base and can put tight control on the bounce rates? How would you handle the bounces? There are several methods to handle bounces, but you need first to understand the basics of bounces, what are the different types of bounces, and how to interpret different bounce error codes.

In this tutorials, you will learn the A-Z of how to handle bounces and what are the different categories of error codes, and how to debug them and fix from the root.

Bounce Email Management

When it comes to email marketing, it is crucial to handle bounces and handling bounce is the process of planning what to do with the email addresses, which has not accepted your messages more than once.

When you talk about sending out an email campaign, it is not for few email addresses, right? The list can have any numbers of email addresses from thousands to millions depending on your customer base. So, let's assume that out of total list about 40% of emails got bounced due to invalid email addresses, then how you will debug those?

Usually, these bounces can be one of that Soft bounce or Hard bounce. These segregations help you in removing the unwanted hard bounced email addresses which don't exist at all.

Every bounce back emails have a message explaining the reason for the bounce. So, you might be thinking then it's pretty easy to read that message and fix the problem. Simple isn't?

I would say NO!  why? Because this interpretation is possible if you are talking about 1-1 or 1-few emails. When it comes to campaigns, practically it won't be scalable enough.

Each bounce back message contains an SMTP error codes and its descriptions. So, either you have to learn these codes, and write programs to handle those or rely on an email service provider like Pepipost, who have already written systems and processes to manage all these bounces for you proactively. 

Almost all ESPs flow their own protocol to interpret these bounces and act. So, it's important to choose the right ESP who has the right set of algorithms to catch these bounces in real-time.

SMTP response codes are the primary source for debugging issues related to bounce. These response codes are the only ways in which email servers communicate over a network. These email servers send out a command like HELO, EHLO, MAIL FROM, DATA and in response receives three-digit code as shown below.

1xx - Informational

2xx - Success

3xx - Redirection

4xx - Persistent transient failure

5xx - Permanent errors

Above are just a small representation for error codes. In this section, you will learn deeply about 4xx - Persistent temporary failure and 5xx - Permanent errors. You can find more at SMTP Error Code library.

Bounce Error Codes and Fixes

Before moving forward with Error code and fixes. Let us make it clear that 5xx = Hard bounce and 4xx = Soft Bounce In the world of SMTP, hard bounces are those emails which can never be delivered, and soft bounces are those emails which couldn't be delivered, but you can try after some time.

441 4.4.1 Error when sending an email

Error Reason: Network is busy! This error states that an email sent to the recipient's email provider was not able to respond to the sender's request.

Solution: The sender's email server will keep on retrying for a few times and pass on further details back to the sender's server. If the sender is unable to deliver it queues up the email and attempt to send it again after a few days.

452 4.2.2 Mailbox Full

Error Reason: An email was not given at recipient MX server because the recipient's mailbox is full.

Solution: These emails can be delivered again, and they send out a message automatically to the sender stating that you should ask your recipient to check or empty their inbox so that new emails can be delivered.

421 4.4.2 - Network Connection Timed Out

Error Reason: The connection between sender and recipient got disconnected before the emails were delivered.

Solution: These responses are immediate when network connections are disconnected so you can always try to send it again after some time.

451 4.4.4 - Temporary server error

Error Reason: The email hasn't failed to deliver due to network problems with the recipient's email provider limit routing the message.

Solution: 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 emails again.

5.0.0 Message Not Delivered: Unknown Issue

Error Reason: The email is undelivered, but the issues are unknown to the server.

Solution: This error states that there might be issues in the email address that can be spelling mistakes or missing letters (symbols). So this emails can be filtered out using universal email regex. You can correct the email addresses and try again. Also, update your contact list with the correct email address. If the problem persists then try contacting your user through a different channel like phone, message or chat to confirm the correct email address.

5.2.0 Other or undefined mailbox status

Error Reason: The email is undelivered to the recipient's mailbox due to an unidentified issue.

Solution: This issue is the same as the above issue since this is as undefined or unknown to the email system. It's better to check the email address syntax if they have some invalid characters or spelling mistake. If this issue persists, try connecting with the user through different mediums.

550 5.2.1 the email account is unreachable

Error Reason: The recipient's address may be removed or temporary disabled for some reason.

Solution: Check with the recipient through the different channel and ask them whether the emails address will be active again. Remove the email from the contact list if email id doesn't exist anymore or else move to the other contact list which you will be sending again after some time.

554 5.2.2 Mailbox full

Reason: The recipient inbox is full and not able to receive new emails.

Solution: Check with your recipient through different channels and ask them to delete from unwanted emails so that some space is free up for new email.

5.3.1 - Email System is not empty

Error Reason: The sender was not able to deliver an email because the email system ran out of space and was not able to store new messages.

Solution: All you need to do is wait for a few minutes and try to send the email again. if this error is persistent, then try to connect your recipient and ask for an alternative way.

5.4.5 - Network Congestion

Error Reason: The email couldn't be delivered, Due to the overloaded network.

Solution: On temporary failure, every email is sent for the retry mechanism. when an email is bounced back after a retry that message within bounce is used for further debugging.

550 5.4.5 Daily user sending quota exceeded

Error Reason: The number of emails you was assigned has crossed the usage limit

Solution: Check with your Email service provider to increase your daily limit. for, e.g. pepipost provides only 100 mails in the free plan but you contact the support and increase your quota.

554 5.4.14 - Hop Count Exceeded 

Error Reason: The email remains undelivered because it has exceeded the routing loop or hop counts for the emails. i.e. the messages are forwarded too many times maybe because of the incorrect user-forwarding/routing tables loop.

Solution: You can check the number of retries a single email has taken. Try sending email again and if an error occurs contact your system administrator to check for configuration in settings.

5.5.3 - Too Many Recipients attached

Error Reason: The sender tried to send email to large no of users at once in such a way that an email system couldn't handle it.

Solution: Segregate your contact list to multiple lists according to the group and send the email one by one. If the error persists, keep segregating your contact list further.

550 5.7.1 - Unable to Relay

Error Reason: the sender couldn't deliver the email because recipient email server has rejected due to some reason which includes system has policy concerns for the specific sender.

Solution: Contact your email support team; it seems like there are some email security issues or policy concern.

554 5.1.0 Sender's Address not allowed

Error Reason: This error occurred while sending an email to the recipient due to invalid recipient's email address.

Solution: Try checking for the invalid characters or spelling mistake within the contact list. If the issues are the fixed update, you list and try to send it again.

550 5.1.1 Recipient address rejected

Error Reason: This error is particular of describing that recipients email address is invalid or does not exist in reality.

Solution: Check your prefix name whether there some invalid characters before the '@' of the email address. If an error exists, try updating the list with a proper email address so that in next retry it is not rejected.

5.1.2 - Host unknown

Error Reason: The Domain Name Of Email Address seems to be Not Valid, i.e. The email domain does not exist or cannot receive emails.

Solution: The email addresses can have symbols or invalid characters after the '@'. i.e. the domain name is incorrect. It can also have the probability that the domain is disabled temporarily.

5.1.3 - Bad destination mailbox address syntax

Error Reason: The email couldn't be delivered because the recipient's email address is not valid.

Solution: Correct the email address, clean the contact list. Check the email address properly by using regex for spaces or unusual characters. If there are any issues, try correcting in the contact list itself.

5.1.4 - Destination mailbox address ambiguous.

Error Reason: The email-ids have multiple address points, i.e. there are multiple inboxes for a single email. so it becomes ambiguous for email system were to relay the message.

Solution: Check with your email administrative whether the email addresses are properly assigned or not.

5.1.6 - Mailbox has moved

Error Reason: The mailbox associated with the Email Address No Longer Accepts Or Forwards Messages. (even though it did in the past). 

Solution: The recipient's email box has been moved to a new destination. The email which was sent previously, the same email can't be sent now. check with your email administrative.

501 5.1.7 - Invalid address in exchange server

Error Reason: The sending address is not a valid email address due to which sender can't deliver an email.

Solution: Check the sender's email address to identify mistakes and missing letters.

550 5.1.8 - Access denied, the bad outbound sender

Error Reason: The email couldn't be delivered because sending address is wrong or can't accept replies. 

Solution: Check the sender's email address to identify spelling mistakes or missing letter within the email address. In configuration, setting, make sure you have entered your email address correctly. If the error persists, try contacting your email administrative team.

552 5.2.3 - Message Is Too Long

Error Reason: The email sent has exceeded the length as compared with the recipient's email system configuration due to which email couldn't be delivered.

Solution: Check the email body should only have the required text and attachments. If possible, try to divide the emails into two or more. so that the emails are delivered without any issue. if Still the problem errors after trimming the email try to contact your email service provider for help.

5.3.0 - Other or undefined mail system status

Error Reason: An unknown problem occurred with more one or more than one recipient's email systems.

Solution: These errors are unclear since the message or error are strange. Starts with checking email addresses syntax. Wait for a few minutes try sending email again. If still, the email is through errors try to contact recipient through alternative mediums and confirm their email address.

5.3.2 - System not accepting network messages

Error Reason: These errors are due to unplanned system maintenance or shutdown or excessive load on an email server.

Solution: you can attempt by sending the same email after some time.

If the problem still arises, contact the recipient and confirm their email address properly.

500 5.3.3 - Unrecognized command

Error Reason: This error states that the email you're sending seems to have a feature which is not supported by recipient's email server.

Solution: Check with sender's email system what new feature they have added to the email configuration which is not supported by the recipient's server. Also, ask for an alternative way for the feature.

552 5.3.4 - Message size exceeds fixed limit

Error Reason: This error states that email message size is above the limit which is configured at the recipient's email server.

Solution: Try to adjust the email message and reduce the attachment size possible. You can also segregate the email into more than one email and then try resending it. If the problem continues, then you might have to review and edit the content (and attachments) thoroughly. 

554 5.4.0 - Unknown Network Connection

Error Reason: When unknown networks communicate between the email server, the email couldn't be delivered.

Solution: Check for an update from email providers regarding network or connection issues. They might be fixing them, and you can try according to the resolution time.

550 5.4.1 Relay Access denied

Error 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.

Solution: The sender's email provider may try resending the message a few more times and provide a further update. Try sending the mail again if the sender's server is not notified within few days of email sent.

5.4.2 - Bad connection

Error Reason: While sending an email from sender's email server to recipient's email server, the network got disconnected before the message was actually handed over for delivery.

Solution: The sender's email program will attempt resending the message a few more times and provide further details.

5.4.3 - Routing server failure

Error Reason: Due to network problem, an email delivery was prevented by the sender's email server.

Solution: The sender's email program will attempt resending the message a few more times and provide further details.

554 5.4.4 Domain Lookup Failed

Error Reason: The email couldn't be delivered due to network issues with the recipient's email provider limit routing the message.

Solution: Check with the email service providers they might be working on resolving such errors. In some time, you can look up your contact list and send your emails.

5.4.6 - Routing loop detected in a network

Error Reason: An email sent was looped retrospectively due to network issues which can use a mail to be forwarded again and again.

Solution: Check that email address is correct and try sending the email back. If that doesn't work, contact your recipient for alternative email.

5.4.7 - Delivery Expired (Message to old)

Error Reason: The email was considered too old by the recipient's email system, and the delay is caused while processing.

Solution: Try to resend the email by checking the recipient's email address and make sure it is correct. If the problem still arises, contact your recipient and ask for an alternative.

5.5.0 - Other or undefined protocol status

Error Reason: The sender's email server couldn't deliver the emails due to unknown technical issues.

Solution: Check your recipient's email address and ensure it is correct and can be resent again. It the problem still exists, contact your email administrative and by that time also ask you, recipient, to provide alternative email id.

5.5.1 - Authentication Required

Error Reason: Due to incorrect messaging command while transmission the email couldn't be delivered

Solution: Try resending the message to identify whether this is a one-time issue or not. if the same occurs again. check your username and password associated with your SMTP. If still, the problem persists, contact your ESP with the issues.

5.5.2 - Syntax error

Error Reason: This error states that the sender's email server issued an unrecognized command

Solution: Try resending the message to identify whether this is a one-time issue or not. if the same occurs again. check your username and password associated with your SMTP. If still, the problem persists, contact your ESP with the issues.

5.5.4 - Transaction failed

Error Reason: This error is the subpart of 5.5.1 and 5.5.2 where a command was used to send an email by sender's server which was unrecognized by the recipient's email server.

Solution: Try resending the message to identify whether this is a one-time issue or not. If the same occurs again. Check your username and password associated with your SMTP. If still, the problem persists, contact your ESP with the issues.

5.5.5 - Wrong protocol version.

Error Reason: the sender's and receiver's email program are using incompatible versions of software due to which an email couldn't be delivered.

Solution: This issue might be temporary or may be resolved to try sending the email again.


Bounce handling is equally important as creating a good looking, customer engaging email campaign. Understanding bounce error is easy, but fixing those might be troublesome. So, its always recommended using the right Email Service Provider, who has the right technology and processes in place to handle all these bounces proactively for you.

If you use your own SMTP to send emails, then you have no option apart from going deep inside the bounce error codes. I hope this tutorial was useful in sharing information about handling bounce back and ways to fix different types of bounce error codes. Feel free to share your experience below in the comments.

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Vikram Sahu ๐Ÿฆ‰

Developer Evangelist, Pepipost

๐Ÿ’ป Senior Software engineer โ†’ ๐Ÿฅ‘ Developer Relations guy โ†’ ๐Ÿค“ Building Community for Email Geeks ๐ŸŽŸ โ†’ Speaker ๐ŸŽ™

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