If those mails are only on the server (that is, you use webmail or imap) only yahoo can restore them for you (maybe).
If those files are in your own local pop3 email system and disappeared there, they only are on your own backup.
I sent some emails back about the start of the new year I had gotten some replies, I needed these saved and they were for awhile in the sent box of my yahoo account. last week i went to view the emails and they are gone? These emails were to a large company and was proof of fraud committed by them, how did they just go away?? I never delete anything from my sent box ever. I heard Gmail will have a feature to send self destruct emails, is this possible in yahoo? or how would one find out if someone got into my account to delete only certain emails? No log in attempts or notices of a strange i.p log attempt.Can i get the emails back in some way?