i just little history about my exchange server. before i have exchange 2003 and server 2003, first we migrate domain 2003 into 2008 sucessfuly and exchange 2003 working fine with new migrated domian. after some days there some hardware issue on exchange 2003 was crash. after that we install exchange on the exsisting domain is working fine. my exchange 2003 was crash on Sep 2011. But today on my exchange i am unable to send the email outside. Internally email working fine and i also able to login exchange outside through owa , but when i send the email outside , emails in the exchange Queue and in Application event i got the error 2159. i attached the image Application Event and also Exchange Queue . please guide me . it is urgent. It is my production server.

Hello This issue seems to be DNS related, Please check DNS can you resolve DNS query. This can be configured on the SMTP sendconnector and the Network tab, check the checkbox "Use External DNS lookup settings on the transport server". Then configure the HUB transport server object and the "External DNS lookup" tab and the "Use these DNS servers" Using your ISP DNS servers as smart host.Thanks Mouzzam Hussain Visit to my Blog mouzzamh.wordpress.com

hi, DNS query failed is the most common error I come across while troubleshooting mail flow for my clients. In this post I will publish all the troubleshooting I have perform to resolve the error. If Exchange server is on windows 2008, please make sure IPv6 is disabled in the network adapter settings.Make sure the deployed firewall is configured to allow DNS port 53 (UDP). If possible configure the same rule in windows internal firewall.DNS server is one of the major component required to run exchange server in your organisation. Therefore, please make sure forwarders and zones are configured properly.Check if HUB server can connect to DNS server. hope can help you thanks,CastinLu TechNet Community Support

Yes there some issue with DNS server. Thanks