Deliverability & the inbox
Soft bounce
Also known as: temporary bounce
A delivery failure the mailbox provider signals as temporary — full mailbox, server down, message too large, temporary rate-limit. Retry is appropriate; continued soft bounces eventually convert to hard-bounce suppression.
A soft bounce is an SMTP 4xx error — the mailbox provider couldn't deliver right now, but the underlying issue is transient. Typical causes: mailbox full, recipient's server temporarily unavailable, message too large, rate-limited by the receiver. ESPs retry soft bounces on a schedule (typically 24-72 hours of retries). Soft bounces are fine in isolation — every send has a small percentage. The risk is chronic: an address that soft-bounces repeatedly for 30+ days should be converted to suppression, because the underlying issue usually isn't resolving. Most ESPs handle this conversion automatically ("soft-bounce-to-hard-bounce threshold") but operators should verify the threshold is set sensibly — seven consecutive soft-bounces over 14 days is a reasonable default.
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