Probably you need a Reverse Record for your Mail Servers in Public DNS to be created. Also try creating SPF Record for your Exchange Server.http://www.msexchange.org/articles/SPF-support-Exchange-freeware.html

Thanks for answer. I already have define A, mx and txt record. my SPF record is "v=spf1 mx -all" However when i check Reverse DNS itdoesnt work. I have added reverse as well as forward zone and addded A record and PTR. But I have two network adapter one is connected to internet through Real Static IP from Hathway and other is connected to LAN through bridge. How can i add reverse DNS entry in my ISP Hathway.