What is a Soft Bounce?
A soft bounce is a temporary email delivery failure where the receiving server acknowledges the email address exists but can't accept the message right now. Unlike hard bounces (which are permanent), soft bounces may resolve on their own, the email might be deliverable if retried later.
Soft bounces are indicated by SMTP response codes in the 4xx range, compared to hard bounces which return 5xx codes.
Why Soft Bounces Matter for Outbound
Soft bounces are less immediately damaging than hard bounces, but they still matter. A high soft bounce rate indicates potential infrastructure issues, and repeatedly soft-bouncing the same addresses wastes sending volume and can accumulate into reputation damage. Most email platforms will retry soft bounces 2-3 times automatically before marking the address as undeliverable.
Common Causes
- Full mailbox: The recipient's inbox has reached its storage limit
- Server downtime: The receiving mail server is temporarily offline
- Message too large: The email exceeds the recipient's size limits
- Rate limiting: The receiving server is throttling your sends
- Greylisting: The server temporarily rejects first-time senders and expects a retry
- Content filtering: The email triggered a temporary content-based filter
Key Differences from Hard Bounces
- Retry eligibility: Soft bounces can be retried; hard bounces should not
- Address validity: Soft bounce addresses are valid; hard bounce addresses are not
- Reputation impact: Soft bounces are less damaging per occurrence, but chronic soft bouncing is problematic
- Action required: Monitor and retry for soft bounces; immediately suppress for hard bounces
Common Mistakes
- Treating soft bounces as hard bounces: Suppressing valid addresses unnecessarily
- Infinite retries: Retrying more than 3 times signals desperation to mail servers
- Ignoring chronic soft bounces: An address that soft bounces on every send should be investigated
- Not tracking soft bounce reasons: The bounce code tells you the exact cause
How FlowStrata Manages Soft Bounces
FlowStrata automatically retries soft bounces up to 3 times with escalating delays. Addresses that soft bounce consistently across multiple campaigns are flagged for investigation and eventually suppressed. We monitor soft bounce patterns to identify systemic issues, like a specific receiving domain throttling our sends, and adjust sending volume and timing accordingly.