Soft Bounces to good email addresses

Soft Bounces to good email addresses

We have had a lot of soft bounces in recent campaigns.
We have telephoned all them and the majority tell us the emaill address we have is correct and they have had no issues with the email box.
Why do we get large numebrs of soft bounces?
We need to understand the problem before three soft bounces tunrs them all into hard bounces.