Exchange and Sendmail

I have an Exchange 2k box and a Linux Sendmail box on the same public network behind my firewall. Both the servers are public. Sendmail cannot send emails to my Exchange. I have added entried in the host files of both and i have also opened relay for the Sendmail box. Still dont get emails from Sendamil. Although both the boxes are sending and receiving emails from everywhere else.

Please help.
Nasir Qadri