Hard bounce vs soft bounce: what is the difference?
By the PickusMail teamUpdated 25 October 20265 min read
Short answer
A hard bounce is a permanent delivery failure: the address does not exist or the domain cannot receive mail, so remove it immediately. A soft bounce is temporary, such as a full mailbox, a busy server or a message that is too large, and the email may be delivered on a later attempt.
The difference at a glance
| Hard bounce | Soft bounce | |
|---|---|---|
| Meaning | Permanent failure | Temporary failure |
| Typical causes | Mailbox does not exist, domain has no mail server, address blocked | Mailbox full, server busy or down, message too large, greylisting |
| SMTP codes | 5xx, e.g. 550 5.1.1 user unknown | 4xx, e.g. 421, 450, 451 |
| Will a retry work? | No | Often, yes |
| What to do | Remove the address immediately | Retry; remove if it keeps failing |
What causes a hard bounce
- The mailbox does not exist. A typo, a made-up address, or someone who left the company.
- The domain cannot receive mail. It has expired, or it has no MX record pointing to a mail server.
- The server refuses you permanently. For example because the sender is blocked.
Mail servers report these with a reply code starting with 5. The most common is 550 5.1.1, meaning the recipient is unknown.
What causes a soft bounce
- The mailbox is full and is refusing new messages for now.
- The server is busy or down, or is limiting how much mail it accepts from you.
- Greylisting. Some servers reject the first attempt from an unfamiliar sender and accept a retry minutes later, to slow down spammers.
- The message is too large for the recipient's limits.
These are reported with codes starting with 4, which tell the sender to try again later.
Why hard bounces hurt more
Sending to addresses that do not exist is the clearest sign of a badly maintained list. Mailbox providers use it to judge whether your mail is wanted, so a high hard-bounce rate pushes more of your email to spam. That is why hard bounces must be removed after the first one. See how to reduce your email bounce rate.
Stop hard bounces before they happen
Email verification finds most future hard bounces before you send: addresses the mail server says do not exist, domains with no mail server, and typos in well-known domains. Check one address free with the email checker, or clean a whole list with PickusMail email verification.
Questions people ask
Should I remove soft-bounced addresses?
Not after one soft bounce. Most sending systems retry automatically. Remove the address if it keeps soft-bouncing across several campaigns.
Can verification prevent hard bounces?
Yes, most of them. Verification finds addresses that do not exist and domains that cannot receive mail before you send, which are the main causes of hard bounces.
Is a full mailbox a hard or a soft bounce?
Usually a soft bounce, because the owner may clear space. If the same mailbox stays full for weeks it is probably abandoned, and you should stop sending to it.
Check your list before you send
PickusMail verifies every address with a real SMTP check and tells you why, in plain words. Unknown results are never charged. Start with 100 free credits.