Too much of a good thing

i'm running os x 10.5.8 on macbook 4.1 , have developed peculiar problem. postage stamp icon parked in usual space in dock displays number thirteen in red circle indicating have 13 new emails. lovely except displays 13 no matter what's in or not in mailbox. how can fix this?

 

quit mail, backup (copy desktop) , delete "com.apple.mail.plist" in macintosh hd / users / / library / preferences / , start mail. did help?

 

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