Soft bounce
Temporary delivery failure that may succeed later — full mailboxes, greylisting, or transient outages.
- Reading time:
- 5 min
- Difficulty:
- Beginner
- Estimated duration:
- 5 min
- Prerequisites:
- None
- Last updated:
- 2026-07-15
- Author:
- empire
Problem summary
Temporary delivery failure that may succeed later — full mailboxes, greylisting, or transient outages.
Severity: Medium
Symptoms
- 4xx or transient DSN statuses
- Mailbox full or try-again-later diagnostics
- Eventual delivery after retries — or eventual hard bounce
Possible causes
- Recipient mailbox quota
- Greylisting / deferral policies
- Transient provider outages
- Rate limiting mistaken for address problems
How to diagnose
- Separate soft-bounce classes from permanent failures
- Track whether soft bounces convert to delivery or hard bounce
- Correlate with deferral and reputation symptoms
How to fix
- Retry with backoff; do not immediately suppress all soft bounces
- Reduce send rate if soft bounces are throttle-shaped
- Suppress addresses that soft-bounce repeatedly over days
How to verify
- Soft-bounce cohort shrinks without elevating hard bounces
- Retry policy matches provider guidance
Prevention
- Prefer engaged addresses
- Avoid blasting cold lists that trigger greylisting
When to escalate
- Production mail is failing for a material share of recipients after remediation attempts
Related Learn Articles
Related Problems
- Deferred messagesReceivers return temporary SMTP failures (4xx), forcing retries instead of immediate acceptance.
- Delayed deliveryMail eventually arrives but takes much longer than normal due to greylisting, queues, or throttling.
- Emails not deliveredMessages never reach the recipient mailbox — rejected, discarded, or stuck without a successful delivery.
- Hard bouncePermanent delivery failure (typically 5xx) because the address or domain cannot accept mail.