Sendmail Relay Issue

My question is that when I set Email Address settings of the Outlook Express in one of the
PC attached with the Server as taisn@yahoo.com, then my Server relays this address because this client PC IP is mentioned in /etc/mail/access file. Of course, it should not do that because taisn@yahoo.com is not the user of my server. How to solve this problem?

Tahir
Pakistan