Understanding Bounce Types: Hard vs Soft
Understanding Bounce Types: Hard vs Soft
Not all bounces are created equal. Understanding the difference determines whether you suppress immediately or retry — and getting it wrong can cost you your sender reputation.
| Hard Bounces | Soft Bounces |
|---|---|
| Permanent failure | Temporary failure |
| SMTP codes 550, 551, 552 | SMTP codes 421, 450, 452 |
| Address does not exist | Mailbox full or server down |
| Remove immediately | Retry then remove |
| Damages reputation heavily | Minor impact if resolved |
Hard Bounces
Permanent delivery failure. Address does not exist or domain is invalid. SMTP codes 550, 551, 552. Remove immediately.
Soft Bounces
Temporary failure. Server down, mailbox full, rate limiting. Codes 421, 450, 452. Retry, then remove if persistent.
DNS Bounces
Domain has no MX records or does not exist. Often indicates typos like @gmial.com.
Block Bounces
Message blocked by reputation, content filters, or blocklists. Requires investigation of sending practices.
Benchmarks
Automation
Configure ESP to auto-handle: hard bounces to suppression immediately, soft bounces retry 3 times then suppress.
Prevention
Verification before sending catches majority of potential hard bounces. Prevention is always more effective than reactive handling.