A soft bounce is a temporary delivery problem an email address randomly encounters. Sometimes, the email message may be too large; the recipient’s email server may be unavailable, or the recipient’s mailbox may be full. All of these lead to a soft bounce. If your email returns a soft bounce message, MailChimp will reattempt sending three times before classifying such as hard bounces.
You may also read: Hard Bounces versus Soft Bounces