Sent Items are showing as read ones in my Yahoo email. Is that normal?

It's a week while I send a message using my Yahoo email, it appears as a read one (unbold) in Sent Items while I have never read them myself. I tought some one may have access to my email; but I don't think so because when I send an email it appears immediately as a read one in sent items, I tried it many times. I have Kaspersky Internet Security 2009 fully updated, so I don't think there is a virus or trojan on my PC. I wonder if it relates to change somthing in Yahoo Options or something else.