What causes soft bounces?

Soft bounces occur when the address is valid but the message cannot reach the mailbox at that moment. The mailbox may be full, often returning an SMTP 452 or 4.2.2 error. The server may be busy or rate limiting. The connection may be temporarily unavailable. Or the domain may be using protective throttling.

Soft bounces often require several retry attempts before the message is marked as failed.

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