Yahoo deleting emails from by sbcglobal account

I still have an sbcglobal email address that I access through yahoo.com.  From time to time my older emails automatically get moved to my trash folder.  Why is this happening?????  I save those emails for a reason.   I don't have a million emails in my inbox.  At any one time I only have about 20.   Is there some limit to emails in your inbox?   How can I stop this fem happening?