Definition
A hard bounce happens when a receiving mail server refuses a message for a reason that will not change if you try again. The classic example is a mailbox that does not exist: the server answers with a 5xx code, often 550 5.1.1 (user unknown), and your platform records a hard bounce.
Other common causes:
- The domain does not exist or has no mail server.
- The address has a syntax error, such as two @ signs or a missing domain.
- The account has been disabled.
- The receiving server has blocked you outright.
The last one is a little different. A block is permanent for that message, but it is about you rather than the address, so the fix is on your side.
Why it matters
Hard bounces are the clearest signal to mailbox providers that a sender does not keep their list clean. A few are normal. A steady stream suggests old, guessed or purchased data, and reputation drops.
Most email platforms suppress an address automatically after a hard bounce. Keep that suppression in place. Re-importing hard-bounced addresses from an old spreadsheet is one of the fastest ways to undo months of good sending.
How SimpleCold handles it
Most hard bounces can be predicted before you send. SimpleCold checks syntax, the domain and its MX records, and then asks the mail server whether the mailbox exists. Addresses that would hard-bounce come back Undeliverable with a reason, for example mailbox_not_found ("This mailbox does not exist.") or no_mail_server ("This domain can't receive email."). These results also carry a failType of permanent, which tells you the failure will not go away on retry.
Remove Undeliverable addresses before a campaign and add them to your suppression list. Run a CSV through the bulk email verifier, or start with the free list cleaning tool.