I am slowly catching up on my reading and found this interesting article about soft and hard bounces hidden deep down in my to read list.
In this article, Derek Harding tells us to put aside convenient yet misleading concepts like hard/soft bounces and look beyond to the untidy reality that is email delivery. Your marketing metrics and your list hygiene will benefit from the simplification and the complexity this entails.
Because there's no agreement on hard and soft bounce definitions. The email delivery protocol, SMTP, classifies failures as transient or permanent. Some people consider a soft bounce to be a transient failure and a hard bounce a permanent failure. This is an excellent, accurate definition, but it doesn't address future deliverability for an email address, which is what marketers usually try to determine with the hard/soft distinction. Also, many email systems don't record transient failures unless they persist for several days, then most are pretty much permanent.
Other people suggest that a soft bounce is a condition such as a full mailbox. Yet such situations are often indicated by SMTP permanent failure codes, as ISPs consider these to be permanent. For example, Yahoo's delivery instructions specifically cite "mailbox full" as a permanent condition that should cause removal of an address from a mailing list. Read the full article here.