Soft Bounce
A temporary email delivery failure (SMTP 4xx), such as a full mailbox or transient server error, that should be retried.
A soft bounce is a temporary SMTP rejection (a 4xx code): mailbox full (452 4.2.2), greylisted (451 4.7.1), connection timed out, or recipient server unavailable. The sending MTA should retry on a backoff schedule (typically up to several days) before giving up. Soft bounces that persist eventually convert to hard bounces in most ESP tracking. A spike in soft bounces from one provider often signals greylisting of a new IP or a reputation issue that has not yet escalated to outright blocking.