Soft bounce
A temporary delivery failure — mailbox full, server temporarily down, etc. Can be retried.
A soft bounce is a temporary email delivery failure. The receiving server has accepted the message but reported a transient problem: the mailbox is full, the server is temporarily down, the message is too large, or the recipient's spam filter is rate-limiting. Soft bounces can be retried; most ESPs retry automatically for 24–72 hours.
Soft bounces are normal — every list has them. The issue is when a soft bounce becomes chronic for a given address (the mailbox is consistently full, the server is consistently unresponsive). Most ESPs convert addresses that soft-bounce repeatedly into effective hard bounces after a threshold (typically 5–10 consecutive soft bounces) and suppress them from future sends.
Spikes in soft-bounce rate usually indicate a specific issue rather than a list-quality problem: a major ISP throttling your IP, a content-size issue (oversized images, multipart bodies), or a sudden volume spike triggering temporary rate-limits. Investigate spikes promptly; they're often the early signal of a deliverability incident.