Email Bounce Types
Hard Bounce vs Soft Bounce
Section titled “Hard Bounce vs Soft Bounce”| Aspect | Hard Bounce | Soft Bounce |
|---|---|---|
| Type | Permanent failure | Temporary failure |
| Cause | Mailbox doesn’t exist, domain error | Mailbox full, server down, rate limited |
| Action | Remove address immediately | Retry, remove after repeated failures |
| SMTP codes | 550, 551, 553 | 421, 450, 452, 552 |
| Impact | High — damages reputation quickly | Low — normal in small quantities |
| Example | ”User unknown” or “No such mailbox" | "Mailbox full” or “Try again later” |
What Is a Good Email Bounce Rate?
Section titled “What Is a Good Email Bounce Rate?”A good email bounce rate is below 2%. Bounce rates between 2% and 5% indicate list quality issues that need attention. Rates above 5% can damage sender reputation with inbox providers like Gmail and Yahoo, leading to lower inbox placement for all future emails from your domain.
How to Handle Bounces
Section titled “How to Handle Bounces”Hard Bounces
Section titled “Hard Bounces”- Remove the address from your mailing list immediately
- Add it to your suppression list
- Never retry sending to a hard-bounced address
- Investigate if you see a spike — it may indicate a list quality problem
Soft Bounces
Section titled “Soft Bounces”- The email service retries automatically (RelayPost retries up to 3 times)
- If an address soft-bounces repeatedly (3+ times), treat it as a hard bounce
- Monitor soft bounce rates — a sudden increase may indicate a server issue
Common SMTP Bounce Codes
Section titled “Common SMTP Bounce Codes”| Code | Type | Meaning |
|---|---|---|
| 550 | Hard bounce | Mailbox not found — address doesn’t exist |
| 551 | Hard bounce | User not local — forwarding error |
| 553 | Hard bounce | Invalid mailbox name |
| 421 | Soft bounce | Service temporarily unavailable |
| 450 | Soft bounce | Mailbox temporarily unavailable |
| 452 | Soft bounce | Insufficient storage — mailbox full |
| 552 | Soft bounce | Message size exceeds limit |
Frequently Asked Questions
Section titled “Frequently Asked Questions”What is the difference between a hard bounce and a soft bounce?
Section titled “What is the difference between a hard bounce and a soft bounce?”A hard bounce is a permanent delivery failure — the email address does not exist or the domain is invalid. A soft bounce is a temporary failure — the mailbox is full, the server is down, or the message was rate limited. Hard bounces require immediate address removal. Soft bounces are retried automatically.
What is a good email bounce rate?
Section titled “What is a good email bounce rate?”A good email bounce rate is below 2%. Rates between 2% and 5% indicate list quality issues. Rates above 5% can damage sender reputation with inbox providers, leading to lower inbox placement for all emails from your domain.
Should I remove soft-bounced addresses?
Section titled “Should I remove soft-bounced addresses?”Not immediately. Soft bounces are temporary and the email service retries automatically. However, if an address soft-bounces repeatedly (3 or more times across different sends), treat it as a hard bounce and add it to your suppression list.