What are common hard bounce reasons?
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A hard bounce is a permanent rejection. The receiving server tells your ESP "this delivery failed and it's not going to work if you try again." These always come back with 5xx SMTP response codes, meaning a final, unrecoverable failure rather than a temporary one (which would be a 4xx).
The most common hard bounce reasons, and what each one means:
- User unknown / Mailbox does not exist (550). The most frequent one. The address either never existed, was a typo at signup, or the account has been deleted. Seeing a lot of these points to stale data or weak acquisition practices.
- Invalid domain / No MX records. The domain part of the address doesn't exist in DNS or isn't configured to receive email. Expired company domains, mistyped domains (gmial.com), and domains that were never set up for email all produce this error.
- Address disabled (551, 553). The account existed but has been disabled by the provider. Common with corporate email when someone leaves a company and IT deactivates the account.
- Syntax error (553). The address is formatted incorrectly in a way the receiving server won't accept. Usually caught by validation before send, but occasionally slips through.
- Permanent policy rejection (550, 554). The receiving server has a permanent block in place for your sending domain or IP. This is different from a temporary rate limit. If you see this repeatedly from the same domain, it's a reputation signal worth investigating.
Your ESP's bounce report will usually include the actual SMTP response code and the error message text. The message text is often more informative than the code alone, because providers like Gmail and Outlook include specific rejection reasons in plain language.
If you're seeing a lot of user-unknown bounces on an older list, that's normal list decay. Email addresses churn at roughly 20-25% per year. If you're seeing domain-related bounces, that's usually a validation or acquisition problem. And if you're seeing policy rejections, that's a reputation signal that warrants a closer look at your authentication setup and sending history before your next campaign.
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