How email bounces work: hard vs soft bounces
A bounce is a rejection notice from the receiving server. Hard and soft bounces mean very different things, and you should treat them differently.
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When you send an email and it doesn't arrive, the receiving server usually sends back a message explaining why. That message is a bounce. Bounces come in two kinds, and treating them the same is a common mistake.
Hard bounces: permanent
A hard bounce means the message can never be delivered to that address. Typical causes:
- the mailbox doesn't exist
- the domain doesn't exist or has no mail servers
- the address was mistyped
The server replies with a permanent error, in the SMTP world a 5xx code, often with an enhanced status such as 5.1.1 ("user unknown").
What to do: remove the address immediately. Retrying will only repeat the failure and count against you again.
Soft bounces: temporary
A soft bounce is a temporary failure. The address might be fine, but something stopped delivery this time:
- the mailbox is full
- the receiving server is down or busy
- the message is too large
- the server is greylisting (deliberately delaying unknown senders)
These come back as 4xx codes. Good sending platforms retry automatically over a period of hours.
What to do: let the retries run. If the same address soft-bounces repeatedly over several campaigns, treat it as dead and suppress it.
Why bounces hurt
Mailbox providers watch how many of your messages bounce. A high hard-bounce rate signals that you collected addresses carelessly or haven't cleaned a stale list. Your sending platform may warn you, throttle you or suspend your account, and mailbox providers will trust you less.
Hard bounces are the ones to prevent, because they are almost entirely avoidable.
Prevent them before you send
The cheapest bounce is the one that never happens. Verification catches the main causes of hard bounces ahead of time: malformed addresses, dead domains and, where the receiving server cooperates, missing mailboxes. See what a good bounce rate looks like and how to clean your list step by step.
Quick reference
| Hard bounce | Soft bounce | |
|---|---|---|
| Meaning | Permanent failure | Temporary failure |
| Typical code | 5xx | 4xx |
| Action | Remove now | Retry, then suppress if it persists |
