Soft Bounce
Also known as: Temporary Bounce, 4xx Bounce
A temporary email delivery failure — recipient inbox is full, mail server is down, or message is too large — that may succeed on retry.
Definition
A soft bounce is a temporary email delivery failure. The receiving mail server accepts the connection but refuses to deliver the message for a reason that may resolve later: the recipient's mailbox is full, the receiving server is temporarily down, the message exceeds the recipient's size limit, or anti-spam filtering has greylisted the sender.
Most email service providers automatically retry soft bounces 3-7 times over 24-72 hours before classifying the address as a hard bounce. Soft bounce rates above 5% are a warning sign — they often indicate stale list data, broken DNS, or reputation problems that are about to become hard bounces.
The SMTP response code distinguishes bounces. 4xx codes (e.g. 421, 450) are soft bounces. 5xx codes (e.g. 550, 552) are hard bounces. The message body usually includes a human-readable reason like 'mailbox full' or 'message too large'.
Why It Matters
Ignoring soft bounces is the fastest path to a hard-bounce spiral. Mailbox providers track persistent soft bounces and start treating them as hard bounces, which damages sender reputation. A clean soft-bounce response plan — retry, then suppress addresses that soft-bounce for 5+ consecutive sends — protects deliverability.
The biggest mistake is grouping all bounces together in a single 'bounce rate' metric. Soft bounces have different root causes than hard bounces and need different responses. Treat them separately in your dashboards and your suppression rules.
Examples in Practice
A marketing campaign to 50,000 recipients sees 1,200 soft bounces (2.4%) on first send. The ESP automatically retries 24 hours later; 900 of those succeed. The remaining 300 get retried again 24 hours after that; 200 more succeed. Net soft bounce after 72-hour retry window: 100 addresses (0.2%) — within healthy range.
A SaaS company's transactional email queue starts showing 8% soft bounces overnight. Investigation reveals their sending domain's DKIM record was accidentally deleted by an admin during a DNS cleanup. Restoring the record clears the soft-bounce spike within hours.
A B2B sender exports their CRM list and sends a campaign without list hygiene. Soft-bounce rate is 12% (mostly 'mailbox full' on long-dormant addresses). They add a rule: any address that soft-bounces on 5 consecutive sends gets suppressed. Soft-bounce rate drops to 1.5% within a month.