Mail receiving 100 plus emails but only one shows in inbox

Running Mac OS X V10.7.5 & mail V5.3 and btyahoo ISP.

Mail shows 'incoming' 100plus emails, but only one or two show in my inbox. Anyone else seen this..?

Also when I shut down my imac, mail wont let me close down. I get the alert "The application mail cancelled shut down" I know I can force quit, but why is this happening in the first place..?

I contacted my ISP (btyahoo) they dont know either...

I have submitted feedback.

Any suggestions plese..Gerry