Outlook sending mail by itself

One of my users discovered that people phone him and tell him there is nothing in his mail.

When you go to sent items there is nothing and no indication that he sent it but the other person said he recieved a blank mail at say 12pm and when you go to sent items there are no mails sent out at 12pm

I scanned the mailbox eith mcaffee and the latest dat files and def files

Please can someone help????