cant send mail to internet exchange2000

i have setup an exchange 2000 server,i can email back and forth on my LAN but i can’t send mail to the internet (e.g. yahoo.com or any other internet mail sites). i have a high speed internet at home, w/c i have a cable modem with a router and is connected to my server. i already tried opening the ports on my router, but still fail to send e-mail messages.i check the port 25 on my server and is working fine. anybody who can help me why i cant send email to internet but can send email on my LAN? i heard about DNS issues..from some’s but i dnt know how its been setup.i have configured my server ip address to use the DNS given to me by the my ISP provider, instead of using the ordinary DHCP. thnks