Connecting Exchange 2000 with SendMail

I am trying to setup an Win2k domain controller with Exchange 2000. It needs to talk to a UNIX (Sun Soloria) box running SendMail. Sun box is connected via T1 to our ISP. The Exchange server has no problems routing mail internally. I can’t get it to talk to either the Sun box (inside the firewall) or other boxes via SMTP outside the firewall. What am I doing wrong?