Some SENT emails do not appear in SENT box

I have the latest Mavericks OS x operating system and fully updated. Some email I send do not appear in the SENT box. It is infuriating. I do not know whether they have reached their destination until I get a reply. Somestime I don't get a reply so where has the email gone. I have search all boxes, turned email accounts off and on, shut down my PC but still this problem persists. I have an iPad and a 5s iPhone and this problem does not occur with these devices so it is not account problem. I am with SKY YAHOO. Derek