What are common email bounce messages and what do they mean?
Michael Ko
Co-founder & CEO, Suped
Published 4 Jun 2025
Updated 13 Oct 2025
6 min read
Email bounces are an inevitable part of sending emails. They are essentially automated messages from the recipient's mail server informing you that your email could not be delivered. Understanding these messages is crucial for maintaining a healthy sender reputation and ensuring your legitimate emails reach their intended inboxes.
Ignoring bounce notifications or failing to interpret them correctly can lead to escalating deliverability issues. Each bounce message, often accompanied by an SMTP error code, provides a clue about why the delivery failed. Deciphering these codes allows you to take targeted action to resolve the underlying problems, protect your sender reputation, and optimize your email campaigns.
I'll delve into the most common bounce messages and their meanings, distinguishing between temporary and permanent failures. By the end, you will have a clearer understanding of how to interpret these critical messages and what steps to take to improve your email deliverability.
Decoding email bounce types
Bounce messages can generally be categorized into two main types: soft bounces and hard bounces. The distinction is vital because it dictates how you should manage the recipient email address moving forward. A soft bounce indicates a temporary delivery issue, while a hard bounce signifies a permanent problem, meaning the email will never be delivered to that address.
A hard bounce typically occurs because the email address is invalid, doesn't exist, or has been permanently blocked by the recipient server. Continuing to send emails to addresses that hard bounce can seriously harm your sender reputation, leading to your emails being marked as spam or blocked by other mail servers.
On the other hand, a soft bounce suggests a temporary problem. This could be due to a full mailbox, the recipient's server being temporarily down, or the email message being too large. You might retry sending the email later for a soft bounce, as the issue may resolve itself. However, repeated soft bounces to the same address could indicate a more persistent problem and should be treated cautiously.
Hard bounce vs. soft bounce
Hard bounce: Permanent delivery failure, such as an invalid or non-existent email address. These should be removed from your list immediately to protect your domain reputation. Learn more about hard bounces.
Soft bounce: Temporary delivery failure, like a full mailbox or server issue. You can retry sending. However, frequent soft bounces to the same address may indicate a larger problem that needs addressing. For more, see the difference between hard and soft bounces.
Common SMTP bounce codes explained
Most bounce messages include a three-digit SMTP code, followed by a more descriptive message. These codes are standardized, although the accompanying text can vary slightly between mail servers. Understanding these codes is key to diagnosing the problem. For example, codes starting with 4xx generally indicate a temporary failure (soft bounce), while 5xx codes point to permanent failures (hard bounces).
SMTP code
Meaning
Type of bounce
550 5.1.1
Recipient mailbox doesn't exist. Often includes NoSuchUser. This is a definitive sign of an invalid email address. Learn more about Google's 550 5.1.1 error message.
Hard bounce
550 5.7.1
Relay access denied or mail content denied. This can point to an issue with authentication or content filtering. Sometimes, this is also seen as a general message for policies like Vade's use of 550 5.7.1.
Hard bounce
550
Generic permanent failure. "Requested action not taken: mailbox unavailable" or "Ip frequency limited". See QQ Mail's specific messages for IP frequency limits and for mail content denied.
Service not available. A temporary issue, possibly related to the recipient server being down or overwhelmed.
Soft bounce
450
Mailbox unavailable or busy. The recipient's mailbox is currently full or experiencing a temporary problem. This is a common soft bounce reason.
Soft bounce
451
Local error in processing. The server encountered a problem but might be able to deliver later. This could be due to a temporary DNS issue.
Soft bounce
452
Insufficient system storage. The recipient's server lacks space to accept the message. It's a temporary issue.
Soft bounce
Beyond these common codes, specific mail providers may issue their own unique messages or sub-codes for various temporary or permanent issues. For instance, Gmail might include additional alphanumeric strings in its bounce messages, as seen in the examples from our Slack discussions. It is important to look at the entire bounce message for the most comprehensive understanding. Further details on bounce message error codes can provide deeper insights.
Example of bounce messages from mail serverssmtp
smtp;550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. Learn more at https://support.google.com/mail/?p=NoSuchUser a11si4116456qtd.334 - gsmtp
smtp;550 Ip frequency limited. http://service.mail.qq.com/cgi-bin/help?subtype=1&&id=20022&&no=1000725
smtp;554 5.7.1 : Relay access denied
Underlying reasons for email bounces
Beyond the technical codes, understanding the root causes of bounces is crucial for prevention. One of the most common reasons for hard bounces is an invalid email address. This can happen due to typos during signup, old email addresses that are no longer active, or emails collected from unreliable sources.
Temporary issues leading to soft bounces often include a recipient's mailbox being full, a server experiencing downtime, or specific message content triggering a temporary filter. While these are temporary, a pattern of soft bounces to the same address may indicate a deeper problem or that the recipient is no longer actively using that mailbox.
Another significant cause of bounces, both soft and hard (or even outright blocks), stems from the recipient server blocking incoming email. This can be due to a poor sender IP reputation, being listed on a blacklist (or blocklist), or issues with your email authentication protocols like SPF, DKIM, or DMARC. Improperly configured DMARC, for example, can lead to authentication failures and subsequent bounces or rejections.
Strategies for reducing bounce rates
To effectively reduce bounce rates and improve email deliverability, a multi-faceted approach is necessary. Regularly cleaning your email list to remove inactive or invalid addresses is paramount. Implementing a double opt-in process for new subscribers can also significantly reduce invalid addresses from entering your list in the first place.
Common issues causing bounces
Outdated lists: Sending to email addresses that are no longer active or have become invalid over time. Regular list hygiene prevents this.
Poor sender reputation: Frequent complaints or previous spam reports can lead to servers blocking your emails.
Monitor blocklists: Actively check if your IP or domain is on any email blacklist (or blocklist), and take steps to delist if necessary.
Implementing robust email authentication, particularly DMARC, is critical. DMARC provides visibility into your email ecosystem and allows you to enforce policies that prevent unauthorized use of your domain, which can inadvertently lead to bounce issues if not properly configured. Use a tool like Suped's DMARC monitoring to get comprehensive insights into your email authentication status and aggregate reports.
Finally, monitoring your bounce reports is an ongoing process. Regular review allows you to identify trends, pinpoint specific problematic recipient domains or email addresses, and adjust your sending practices accordingly. This proactive approach ensures you maintain high deliverability rates and a strong sender reputation.
Views from the trenches
Best practices
Always implement double opt-in for new subscribers to verify email addresses and prevent bounces.
Regularly clean your email lists by removing hard bounced addresses to maintain sender reputation.
Monitor your DMARC reports closely to identify authentication failures that might lead to bounces.
Segment your audience effectively to send relevant content and reduce unengagement related bounces.
Warm up new sending domains or IPs gradually to build a positive sending history with ISPs.
Common pitfalls
Ignoring bounce notifications leads to damaged sender reputation and higher email rejections over time.
Sending to unengaged or outdated lists significantly increases your hard and soft bounce rates.
Failing to configure SPF, DKIM, and DMARC correctly results in authentication failures and blocked emails.
Not monitoring email content for spam trigger words can cause emails to be filtered or bounced.
Expecting instant deliverability success on a new domain without a proper warming-up strategy.
Expert tips
Use a dedicated sending domain for transactional emails to separate their reputation from marketing emails.
Analyze bounce messages for specific error codes to diagnose issues beyond just soft vs. hard categories.
Invest in an email validation service to proactively identify and remove invalid addresses from your list.
Leverage DMARC reporting tools to gain granular insight into bounce causes and authentication status.
Periodically review your email sending volume and frequency to avoid triggering rate limits from ISPs.
Marketer view
Marketer from Email Geeks says that new domains need a proper warm-up strategy, suggesting that my high bounce rates on abandoned cart and thank you flows could be due to the domain only being 30 days old.
2022-09-26 - Email Geeks
Expert view
Expert from Email Geeks says that the specific bounce messages are critical for troubleshooting, emphasizing that 'soft' and 'hard' bounces alone don't provide enough context.
2022-09-26 - Email Geeks
Mastering email bounce management
Email bounce messages are more than just delivery failures; they are invaluable feedback signals that, when understood and acted upon, can significantly enhance your email deliverability. By differentiating between soft and hard bounces, deciphering SMTP error codes, and addressing the root causes of delivery issues, you can maintain a clean email list and a strong sender reputation.
Proactive steps like validating email addresses, implementing robust authentication (SPF, DKIM, DMARC), and continuously monitoring your bounce reports are essential for email marketing success. Embracing these practices will ensure your messages reliably reach the inbox, fostering better engagement and campaign performance.
