When an email can't be delivered, it bounces — but not all bounces mean the same thing. A hard bounce is a permanent failure: the address doesn't exist, the domain is invalid, or the server has blocked you for good. A soft bounce is temporary: a full mailbox, an oversized message, or a server that's briefly down.
Mixing them up damages your sender reputation. Hard bounces must be suppressed immediately; soft bounces can be retried. Mailbox providers watch how you react, so handling each correctly protects deliverability.
At a glance
Aspect
Hard Bounce
Soft Bounce
Nature
Permanent failure
Temporary failure
Typical cause
Invalid or nonexistent address
Full inbox, server down, message too big
Right response
Suppress immediately
Retry, then suppress if persistent
Reputation impact
High if repeated
Lower, unless chronic
Retry?
No
Yes, for a limited window
When to use Hard Bounce
You see permanent failure codes (5xx) or unknown-user errors.
The address or domain is clearly invalid.
You need to add the address to a suppression list now.
When to use Soft Bounce
You get temporary failure codes (4xx) like mailbox full.
The recipient server is momentarily unavailable.
You want to retry before giving up on the address.
Verdict
Suppress hard bounces on the first failure — continuing to mail them is the fastest way to tank your reputation. Retry soft bounces a few times over a short window, but convert them to suppressions if they keep failing. The best defense against both is verifying and cleaning your list before you send, not after the damage is done.
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