What are common email bounce messages and what do they mean?
Michael Ko
Co-founder & CEO, Suped
Published 4 Jun 2025
Updated 16 Aug 2025
6 min read
Sending emails is a fundamental part of communication, but sometimes your messages don't reach their intended recipients. When an email fails to deliver, the sending server often receives an automated notification, commonly known as a bounce message or Non-Delivery Report (NDR). These messages are critical because they explain why your email couldn't be delivered.
Understanding these messages is key to maintaining a healthy sender reputation and ensuring your email campaigns are effective. Ignoring bounces can lead to higher bounce rates, poor inbox placement, and even land your sending IP or domain on a blocklist (or blacklist).
This guide will help you decipher common email bounce messages, explaining what they mean and how you can address them to improve your overall deliverability.
Understanding bounce messages and codes
Email bounce messages are typically structured using SMTP (Simple Mail Transfer Protocol) codes, which are three-digit numbers that provide a standardized reason for delivery failure. The first digit indicates whether the failure is permanent or temporary. A 4xx code signifies a temporary problem (a soft bounce), meaning the mail server might try to resend the message later. A 5xx code indicates a permanent failure (a hard bounce), where the message will not be delivered.
Beyond these codes, bounce messages often include a human-readable explanation that offers more specific details about the issue. This descriptive text is crucial for diagnosing the exact problem. For a comprehensive overview of these codes, refer to our guide on SMTP bounce codes and their meanings.
While SMTP codes provide the technical classification, the accompanying text clarifies the root cause. For example, a 550 code could mean the recipient address doesn't exist, or it could mean the message was blocked by a spam filter. Understanding both parts of the bounce message is essential for effective troubleshooting.
Invalid command syntax, typically indicates a sending server configuration issue
Common hard bounce messages
Hard bounces are permanent delivery failures, meaning the email will never be successfully delivered to that address. These are critical signals that the recipient's email address is invalid or no longer exists. For more details, explore the nuances of hard and soft email bounces.
One of the most frequent hard bounce messages is "550 5.1.1 The email account that you tried to reach does not exist". This indicates the recipient's email address is either misspelled or has been deactivated. Sending to non-existent addresses severely damages your sender reputation. For more information on this specific type of bounce, see the Google Mail help page.
Another common hard bounce is related to the domain itself, such as "No MX record for domain" or "Host unknown". This means the recipient's domain doesn't exist or isn't configured to receive email, often due to a typo in the domain name. It’s important to understand the implications of a ‘no MX’ bounce and its causes.
Action required
Always remove email addresses that generate hard bounces from your mailing lists immediately. Continued attempts to send to these addresses will harm your sender reputation and could lead to your emails being flagged as spam for valid recipients.
Common soft bounce messages
Soft bounces, on the other hand, are temporary delivery failures. These messages typically indicate that the email reached the recipient's mail server, but it couldn't be delivered for a transient reason. Unlike hard bounces, the sending server will usually retry sending the email for a certain period. To understand soft bounces in more detail, refer to our article on how to fix soft bounces.
A common soft bounce message is "552: Mailbox full" or "Message too large". This means the recipient's inbox has reached its storage limit, or your email (including attachments) exceeds the recipient server's size limit. Reducing email size or waiting for space to clear up can resolve these issues.
Other soft bounces, often indicated by 4xx codes, include "Temporary server error" or "IP frequency limited". These can be due to network issues, a recipient server being temporarily unavailable, or your sending IP hitting rate limits. Learn more about common bounce codes and their meanings on WP Mail SMTP's guide to email bounces. While these are temporary, a high volume of soft bounces can still signal underlying deliverability issues.
Hard bounces
Permanence: Indicates a permanent error. Email address is invalid or non-existent.
Action: Immediately remove the recipient from your mailing list. Continued sending harms sender reputation.
Impact: High hard bounce rates lead to lower sender scores and potential blocklisting (or blacklisting).
Soft bounces
Permanence: Indicates a temporary error. Email may be delivered later.
Action: Monitor if the issue persists. Multiple soft bounces to the same address may indicate it's effectively a dead address.
Impact: While less damaging than hard bounces, high rates can still affect deliverability and signal issues with your list or sending practices.
Server and content related bounce messages
Beyond recipient-specific issues, bounce messages can also signal problems with the receiving mail server's policies, your sender reputation, or even the content of your email. Messages like "554 5.7.1: Relay access denied" or "Message refused" often fall into this category. These typically mean the receiving server actively blocked your email. This might be due to your IP address or domain appearing on a public or private blacklist (or blocklist), or because your email triggered spam filters. You can find more insights into specific issues like 550 5.7.1 and 554 errors in our troubleshooting guide.
Being listed on an email blacklist (or blocklist) is a serious concern for deliverability. Mailbox providers use these lists to identify and reject email from known or suspected spam sources. If you receive bounce messages indicating your email was refused due to an IP or domain block, it's crucial to check your blocklist status. Our in-depth guide to email blocklists provides comprehensive information on this topic.
Finally, "Mail content denied" messages suggest that the content of your email triggered a spam filter. This could be due to suspicious links, excessive capitalization, too many emojis, or other characteristics commonly associated with spam. Reviewing your email copy and design for potential spam triggers is important in these cases.
Example of a blocklist-related bounce message
smtp;554 5.7.1 Service unavailable; Client host [203.0.113.42] blocked using bl.spamcop.net; See: http://www.spamcop.net/bl.shtml?203.0.113.42
Views from the trenches
Best practices
Actively monitor your bounce rates and analyze bounce messages regularly to identify recurring issues.
Segment your audience to exclude recipients who consistently generate hard bounces.
Implement a double opt-in process for new subscribers to ensure valid email addresses.
Warm up new sending domains and IPs gradually to build a good sending reputation.
Common pitfalls
Ignoring hard bounces and continuing to send to invalid addresses, which damages sender reputation.
Not analyzing specific bounce error codes and generic messages, hindering effective troubleshooting.
Sending a high volume of emails from a new domain or IP without proper warming up.
Using overly promotional language, excessive links, or too many emojis that trigger spam filters.
Expert tips
Use email validation services to clean your lists periodically, proactively removing invalid addresses.
Leverage DMARC reports to gain insights into delivery issues and authentication failures.
Maintain consistent sending volumes and frequencies to establish a predictable sending pattern.
Prioritize engagement: send relevant content to engaged subscribers to improve inbox placement.
Expert view
Expert from Email Geeks says: Understanding specific bounce messages is crucial because 'soft' and 'hard' labels alone don't provide enough detail for effective troubleshooting. You need to look at the full error code and descriptive text.
2022-10-01 - Email Geeks
Marketer view
Marketer from Email Geeks says: When facing high bounce rates, especially on automated flows like abandoned carts or welcome series, it's important to analyze the specific bounce messages. Factors like recent domain purchase or low initial sending volume can contribute to these issues.
2022-10-01 - Email Geeks
Taking action on bounce messages
Email bounce messages are more than just delivery failure notifications; they are crucial feedback loops that inform your email deliverability strategy. By understanding the common types of bounce messages and their underlying causes, you can proactively clean your email lists, optimize your sending practices, and ultimately ensure your messages reach the intended inboxes. Regularly monitoring and acting on these insights is fundamental to maintaining a strong sender reputation and achieving successful email campaigns.
