A soft bounce indicates a temporary delivery issue of the campaign to an email address. This can be due to the email inbox being full, or the recipient mail server is experiencing some technical issues and is temporarily offline for example. In short this means the current campaign could not be currently delivered, but if you try again in the future you might be able to deliver the message because this is not a permanent delivery issue.

A hard bounce however means that there is a permanent issue with the delivery. This is due to email address not existing anymore for example or maybe the domain name of the recipient has expired…