A soft bounce is an email sent to an active email address which is turned away before being delivered. The problem is normally temporary.
A soft bounce may happen when a server is down or the recipient's mailbox is full. The email might be held at the recipient's server and delivered later, or the sender's email program may attempt to deliver it again. Typically, soft-bounced emails are identified with a 400-series (4XX) Simple Message Transfer Protocol (SMTP) reply code.