every mail u send through outlook express get recorded in log file in window folder with name smtp.log (i do not remember the name of the log file exactly ) .
content is not logged but all the command used to send the mail by ur e-mail client to tranfer ur mail to the server gets logged which includes e-mail headers of the person sending the mail as well as the person to whom the mail is being sent.
even if u delete the mail from ur ur inbox ,the log file remains there which is a security threat also as someone can easily gain information about all the sent and recieved.
so u go though that log if they are not deleted.