Sendmail become open relay

[lyd mc]

Hi guys need help..

My mailserver become an open relay.

Unknown user can now send mail.

snippet from mailq

m88C8iWq042874      689 Mon Sep  8 20:08 <osxch at mail.mydomain.com>
                 (Deferred: Name server: mx1.mail.tw.yahoo.com.: host name loo)
                                         <chenaa00 at yahoo.com.tw>
                                         <chena0.tw at yahoo.com.tw>
                                         <chena0877 at yahoo.com.tw>
                                         <chena0 at yahoo.com.tw>
                                         <chena11 at yahoo.com.tw>
                                         <chena121959330 at yahoo.com.tw>
                                         <chena1238 at yahoo.com.tw>
                                         <chena186890 at yahoo.com.tw>
                                         <chena1966 at yahoo.com.tw>
                                         <chena20155 at yahoo.com.tw>
                                         <chena226 at yahoo.com.tw>
                                         <chena22 at yahoo.com.tw>
                                         <chena26232000 at yahoo.com.tw>

I don't  have user 'osxch' and there others can also send..


best regars thnx

alydio