All my mail is coming in as having been read when it has not been read! What can I do?

I use Yahoo mail. I have a pc and a mac. Yesterday I tried to email on the mac and it asked to set up Mail there. When I did, it transfered all my yahoo emails into mac mail, tho I still have yahoo mail and it works the same - with the exception: ALL EMAILS COME IN AS ALREADY READ (not bold, unread) and this is making it very difficult to see new emails. Any suggestions how I did this and how I can undo it?