Soft bounce vs hard bounce: what is the difference?
A bounce is an email that was not delivered. Whether it is "soft" or "hard" tells you if trying again could ever work.
A hard bounce is permanent: the address or domain does not exist, or the server refuses you for good (error codes starting with 5). Remove it right away. A soft bounce is temporary: a full mailbox, a busy server or a rate limit (codes starting with 4). The system retries it; remove the address if it keeps soft-bouncing.
The difference at a glance
| Detail | Hard bounce | Soft bounce |
|---|---|---|
| Meaning | Permanent failure | Temporary problem |
| Code class | 5.x.x (for example 550) | 4.x.x (for example 421, 450, 451) |
| Common causes | Address does not exist, domain does not exist, blocked by policy | Mailbox full, server busy, sending too fast, temporary block |
| What to do | Remove the address now | Let it retry; remove after repeated soft bounces |
The standard for these codes (RFC 3463) says it plainly: a 4 means a temporary failure where resending may work later, and a 5 means a permanent failure where resending the same message will not.
Common bounce codes and what they mean
| Code | What it means | Type |
|---|---|---|
| 550 5.1.1 | The recipient address does not exist | Hard |
| 550 5.7.1 | Blocked by the receiver's policy or spam filtering | Hard |
| 550 5.7.26 | The sender failed authentication (SPF, DKIM or DMARC) | Hard |
| 552 5.2.2 | At Gmail: the mailbox is full and inactive | Hard at Gmail (the 452 4.2.2 version is temporary) |
| 421 4.7.0 | Temporarily rejected, often over sender reputation or setup | Soft |
| 450 4.2.1 | The recipient is receiving mail too fast | Soft |
| 451 4.3.0 | Temporary error at the receiving server | Soft |
Codes can mean different things at different providers. In Microsoft 365, for example, 5.2.2 can mean the sender hit a sending limit. Always read the text next to the code.
What email platforms do automatically
- Mailchimp removes hard-bounced addresses right away, and turns repeated soft bounces into a hard bounce (after 7 for contacts with no activity, or 15 for contacts with past activity).
- HubSpot stops sending marketing email to hard-bounced contacts, keeps soft-bounced contacts eligible, and retries delivery for up to 72 hours.
How to prevent bounces
- Verify the list before you send, especially if it is older than a few months. See how to clean an email list.
- Fix typos at sign-up (gmial.com, yaho.com).
- Use double opt-in so only real, working addresses join.
- Set up SPF, DKIM and DMARC so you are not refused as an unauthenticated sender.
- Keep your bounce rate at or under about 2%. Above 5% is a serious warning sign.
Questions people ask
- What does "bounced email" mean?
- The message was returned as undeliverable. You usually get a message back explaining why, with an error code.
- Is a full mailbox a hard or soft bounce?
- Usually soft (4.2.2). Gmail returns a hard 552 5.2.2 when the mailbox is full and the account looks inactive.
- Should I resend to a soft bounce?
- Your email platform retries automatically. If the same address soft-bounces on several sends in a row, remove it.