EXTERNAL WEB AND EXCHANGE SERVER

DEAR FRIENDS
RECENTLY I CONFIGURED ONE WEB AND EXCHANGE SERVER IT IS WORKING FINE IN INTERNAL NETWORK
WHEN I CONNECTED TO INTERNET WEB SERVER IS WORKING BUT MAIL DIDNT RESPONDING.WE HAVE ONE GLOBEL IP AND 64 IPS FROM ISP.WE CONNECTED DLINK ROUTER AND USING NAT FOR TRANSLATING INTERNAL IPSTO EXTERNAL.WE MAINTAINING OUR OWN DNS HIRE.AND ENTRIES R ENTERED FOR EXCHANGE BUT STILL ITS NOT WORKING .CAN U TELL ME WHAT ENTRIES HAVE TO DO IN DNS.