Suddenly my iMac puts in a red indicator at the bottom of my desktop not how many unread emails I have, but the TOTAL of emails in my system. That makes the number useless.

Suddenly my iMac puts in a red indicator on my desktop not how may new, un-read emails I have, but the TOTAL of emails in my system! That makes the number useless. It used to let me know how many emails I needed to deal with. Now it’s a useless figure (except telling me that I should clean up the backlog!) 

How do I get it back to counting the NEW emails?