An email bounce, or bounce back, is a message that could not be delivered, returned to the sender with an explanation. If you have ever received a "Delivery Status Notification", a "Mail Delivery Subsystem" message or an "Undeliverable" notice, you have seen one. Bounces are normal in small numbers and a serious warning in large ones. This guide explains what an email bounce is, the difference between hard and soft bounces, the most common reasons for a bounce back, how to read the message, how to fix each cause and how to keep your bounce rate low.
What is an email bounce?
When you send a message, your server hands it to the recipient's server. If that server refuses the message, either at once during the conversation or later after accepting it, a failure notice is generated. The notice goes to the return-path address of the original message. The notice is called a bounce, a non-delivery report or a delivery status notification. It normally contains an SMTP reply code, an enhanced status code and a line of text explaining the problem.
Not every message that fails is a bounce in the strict sense. A message that is accepted and then placed in the spam folder is not a bounce, because nothing was returned. For the many ways mail can go missing without any notice, see why emails won't send.
Hard bounce vs soft bounce
| Hard bounce | Soft bounce | |
|---|---|---|
| Meaning | Permanent failure | Temporary failure |
| SMTP code range | 5xx | 4xx |
| Typical causes | Address does not exist, domain does not exist, message blocked for policy reasons | Mailbox full, server busy, message too large, greylisting |
| What to do | Remove the address and do not retry | Retry for a while; remove the address if it keeps failing |
Good sending software retries soft bounces on a schedule before giving up, and suppresses hard bounces automatically so that you never send to them again. The two classes are defined by the SMTP standard, and our guide to SMTP bounce codes goes through the codes in detail. Soft failures are also explained in queued and deferred email.
Common reasons for a bounce back
1. The address does not exist
The most common reason is a typo, a person who has left a company or an address that was never real. The status is usually 5.1.1 with a message such as "user unknown" or "recipient address rejected". Remove the address.
2. The domain does not exist or cannot receive mail
The domain is misspelled, has expired or has no mail server records. The message may say that the host or domain was not found. Check the spelling, and if the address is correct, the domain really is unreachable.
3. The mailbox is full
The recipient has run out of storage. This is usually a soft bounce, with a status like 4.2.2 or sometimes 5.2.2. Retrying later may work. If the mailbox stays full for days, the person is probably not using it.
4. The message is too large
The attachments or images make the message bigger than the recipient's server accepts. Send a link instead of a large attachment.
5. The message was blocked as spam or for policy reasons
The recipient's server did not like the message, the sending IP address or the domain. The text often mentions a blocklist, a policy or a reputation problem, with a status such as 5.7.1. This is a sender-side problem, and it is a hard bounce in effect. Read email blocked message: why and how to fix and email reputation.
6. Authentication failed
Many receivers reject mail that fails SPF, DKIM or DMARC checks, especially for domains with strict policies. The bounce text mentions authentication or a DMARC policy. Fix your records; see authenticating your sending domain.
7. The recipient server is down or busy
Connection timeouts and temporary errors usually clear on their own. The sending server tries again, and the message goes through.
8. Greylisting
Some servers temporarily refuse the first attempt from an unknown sender and accept the retry. This appears as a short delay and not a final bounce.
9. Mail forwarding or auto-reply loops
A forward that points to a dead address, or two auto-responders replying to each other, can produce bounces from an address you never wrote to. See email forwarding explained.
10. Backscatter
If you receive bounces for messages you never sent, someone may have forged your address as the sender of spam. The notices are sometimes called backscatter. Make sure your domain has SPF, DKIM and DMARC, which helps receivers reject forged mail instead of sending bounces to you.
How to read an email bounce message
A typical bounce contains four useful pieces of information.
- The failed recipient address. Check it for typos first.
- The reply code, a three-digit number. Codes starting with 4 are temporary and codes starting with 5 are permanent.
- The enhanced status code, in the form class.subject.detail, such as 5.1.1. The first digit again says temporary or permanent, and the next digits give the reason.
- The human-readable text, which often names the exact cause, a blocklist or a policy.
If the message came from the sender's own mail system, it may be labelled "Mail Delivery Subsystem". Do not confuse a real bounce with a phishing message that imitates one; see Mail Delivery Subsystem: scam or bounce.
How to fix a bounce
- Read the explanation. The text and code usually tell you the cause.
- Check the address. Look for typos, extra spaces or a wrong domain, and try to confirm the address another way.
- Retry soft bounces later. If the cause was a full mailbox or a temporary error, resend after a few hours.
- Shrink large messages. Compress, link to a file or remove images.
- Fix authentication. Make sure SPF, DKIM and DMARC pass for the domain you send from.
- Check blocklists. Run your sending IP and domain through our blacklist check if the bounce mentions blocking.
- Contact the recipient another way if a hard bounce says the address does not exist.
- Contact the recipient's mail administrator if a block seems wrong, and include the exact bounce text and times.
What is the email bounce rate?
The bounce rate is the share of messages sent that bounced. Divide the number of bounces by the number of messages sent and multiply by 100. Most sending platforms report hard and soft bounces separately. We do not give a single target for a good rate, since providers and platforms publish their own thresholds and they change. As a rule, keep hard bounces very low, and treat any sudden rise as an emergency, because mailbox providers read a high bounce rate as a sign of a poor-quality list.
How to prevent bounces
- Collect addresses carefully. Use confirmation on sign-up forms. See double opt-in.
- Validate addresses before you send. Email validation catches typos, non-existent domains and risky addresses, and our free email validator checks the domain and syntax.
- Remove hard bounces immediately. Never send to an address that has bounced permanently.
- Clean old lists before a big send, especially lists that have not been mailed for a long time.
- Never buy lists. Purchased addresses are full of invalid and trap addresses; see spam traps.
- Monitor your reports. Watch bounces by provider and by code, so that you notice a block early. Our email analytics and webhooks report bounces as they happen.
Stop bouncing before you send Validate every address at sign-up and before each campaign, and suppress hard bounces automatically.
References and further reading
Frequently asked questions
What does an email bounce mean?
The message could not be delivered and a failure notice was returned to the sender with a code and an explanation.
What is the difference between a hard and a soft bounce?
A hard bounce is a permanent failure, usually a 5xx code, such as an address that does not exist. A soft bounce is temporary, usually a 4xx code, such as a full mailbox or a busy server.
Why does my email bounce back?
Common causes are a mistyped or non-existent address, a full mailbox, a message that is too large, the message being blocked as spam, failed authentication or a temporary server problem.
How do I fix a bounced email?
Read the bounce text and code, check the address, retry soft bounces later, reduce message size, fix SPF, DKIM and DMARC and check blocklists if the bounce mentions blocking.
How can I reduce my bounce rate?
Confirm addresses at sign-up, validate lists before sending, remove hard bounces immediately, clean old lists and never buy email lists.