Soft Bounce happens when an email cannot be delivered due to a problem in the receiver's server. If the email cannot be reached the targeted inbox within and after 72 hours, it turns out a hard bounce.
Soft bounces typically indicate a temporary delivery issue and are handled differently than hard bounces by Mailchimp. When an email address soft bounces, it will immediately display as a soft bounce in the campaign report.
If an email address continues to soft bounce, the address will be considered a hard bounce in the end and must be cleaned from your email list.
Here, some common reasons for soft bounce are given below;
Mailbox of subscriber is over her quota.
Recipient email server is down or offline.