Hard Bounce vs Soft Bounce: Codes, Causes and What to Do
By Eric Dieperink · Published September 18, 2026 · Last reviewed September 18, 2026
A hard bounce is a permanent rejection (SMTP 5xx code): the address will not accept mail, now or later. A soft bounce is a temporary failure (4xx code): the server could not deliver right now, but the address itself is not necessarily broken. The distinction comes straight from SMTP, and it dictates opposite actions: never email a hard bounce again; give soft bounces time before doing anything.
Key takeaways
Hard bounce = permanent (5xx). Remove and suppress. Never retry the address.
Soft bounce = temporary (4xx). Retry on later sends; only remove after a persistent pattern.
The bounce reason is often not visible: many servers send a generic 550 regardless of the true cause.
Bounce rates are a deliverability signal: mailbox providers watch them, and Google and Yahoo publish explicit expectations for bulk senders.
Cleaning your list before sending prevents most hard bounces. Bounce logs are the expensive way to discover the same dead addresses.
Where the codes come from
Every bounce is an SMTP status reply, standardized in RFC 5321. Codes with the pattern 5xy signal a permanent failure: repeating the same command will fail again. Codes with the pattern 4xy signal a temporaryfailure: the server is asking you to try again later. ESP dashboards add friendly labels (“invalid mailbox”, “mailbox full”), but underneath those labels is always one of these two classes.
Common codes and what they mean
| Code | Type | Typical meaning | Action |
|---|---|---|---|
| 550 | Hard | Mailbox unavailable, not found, or rejected by policy | Remove and suppress the address |
| 551 | Hard | User not local; the recipient moved on | Remove; check for a new address if you have another channel |
| 553 | Hard | Mailbox name not allowed (bad syntax or rejected format) | Remove and suppress |
| 421 | Soft | Service not available; server overloaded or rate-limiting you | Retry later; slow your sending if it repeats |
| 450 | Soft | Mailbox temporarily unavailable (e.g. greylisting) | Retry on a later send |
| 451 | Soft | Local error in processing; try again | Retry on a later send |
| 452 | Soft | Insufficient storage; mailbox full | Retry; investigate if it persists for weeks |
What causes each type
Hard bounces
The mailbox was deleted, or never existed (typo, fake signup, purchased data).
The domain expired or dropped its mail records, which is very common on lists older than a year.
The receiving server blocks your mail by policy (often disguised as a 550).
Soft bounces
Greylisting: the server deliberately defers unknown senders once, then accepts the retry.
A full mailbox or an overloaded server.
Temporary technical problems on either side, or rate limits triggered by sudden volume spikes.
What to do, and what not to do
For hard bounces, the action is final: add the address to your suppression list so no future send includes it. Retrying a hard bounce is one of the fastest ways to look like a spammer to a mailbox provider. For soft bounces, patience is the strategy: most resolve on a retry. Only treat an address as dead after a persistent pattern across multiple sends, then suppress it as well. How suppression lists fit into the broader workflow is covered in what is an email suppression list.
One honest caveat: the bounce text you receive is not always truthful. Some servers return a generic 550 for policy rejections, and aggressive anti-spam filters sometimes reject valid mail. That is why bounce handling should feed your decisions, not replace list-hygiene practices.
Preventing bounces beats processing them
Bounce handling is reactive by definition. The proactive version is the same problem solved earlier: a pre-send cleaning pass removes dead domains, malformed lines, duplicates and disposable addresses before they ever reach a mail server. The full sequence is in how to clean an email list.
Keep reading
How to clean an email list · Verification vs validation vs cleaning
Fewer bounces start before the send
ListGuard removes the causes of most hard bounces in one pass and returns a plain report of what was removed and why. See how the cleaner works or create a free account.
Sources
RFC 5321: Simple Mail Transfer Protocol (4xy/5xy reply code semantics) · Google: Email sender guidelines · Yahoo Sender Hub: Best practices