A hard bounce is an email that is permanently rejected because the recipient address is invalid — the mailbox does not exist, the domain is wrong, or the server has permanently blocked delivery. Unlike a soft bounce, a hard bounce will never succeed, so the address should be removed from your list immediately.
Hard bounce vs soft bounce
A soft bounce is a temporary failure (a full mailbox or a server that is briefly unavailable) and may deliver on retry. A hard bounce is permanent. Repeatedly mailing addresses that hard-bounce signals careless list hygiene to mailbox providers and drags down your sender score.
| Hard bounce | Soft bounce | |
|---|---|---|
| Type of failure | Permanent | Temporary |
| Typical causes | Address or domain does not exist, mailbox disabled, message refused by policy | Mailbox full, server busy or briefly unavailable, message too large |
| Typical SMTP code | 5xx (for example 550 5.1.1) | 4xx (for example 421 or 452) |
| Will retrying help? | No | Often yes: the server may accept it later |
| What to do | Remove the address after the first hard bounce | Retry; remove the address only if it keeps bouncing |
Why hard bounces matter
A high hard-bounce rate hurts deliverability and can get you flagged or blacklisted. Keeping it low — by using confirmed sign-ups and cleaning your list — protects inbox placement. Learn more in our guide to email bounces and the bounce types that matter.
Common hard bounce codes and what they mean
The bounce message usually contains an enhanced status code that tells you why the email was refused:
| Code | Meaning | Most likely cause |
|---|---|---|
| 5.1.1 | Bad destination mailbox | The address does not exist or contains a typo (for example gmial.com) |
| 5.1.2 | Bad destination system | The domain does not exist or has no mail server |
| 5.2.1 | Mailbox disabled | The account has been closed or suspended |
| 5.7.1 | Delivery not authorized | The receiving server refused the message by policy, sometimes because of a blocklist |
For step-by-step fixes, see our FAQs on error 5.1.1 and error 550.
What should you do about hard bounces?
- Stop sending to the address: a hard bounce will not deliver on a second try.
- Fix obvious typos if you can confirm the right address with the contact.
- Collect addresses with double opt-in so invalid ones never reach your list.
- Clean the list before a big send if it has not been used for months. See how to keep your email list clean.
Frequently asked questions
What is a good hard bounce rate? As close to zero as possible. A list built with confirmed sign-ups rarely produces more than a handful of hard bounces per campaign; a sudden jump usually means an old, imported or bought list.
Is a full mailbox a hard bounce? Usually not. Most servers report a full mailbox as a temporary 4xx error, so it is treated as a soft bounce.
Can a hard-bounced address become valid again? Rarely. If a contact tells you the address works again, re-add it with fresh consent rather than removing the block automatically.
How Mailpro handles hard bounces
Mailpro detects hard bounces automatically and stops sending to those addresses, so your stats stay clean and your reputation protected. See how bounce management works. Related: bounce rate and email blacklist.
Related guide: How to manage email lists: 16 best practices
Mailpro and bounce management
Keep bounces from wrecking your deliverability
Mailpro detects and removes hard bounces automatically and shows them in your stats, so your list stays clean and your sender reputation stays high.