Hi forum members, I need a solution for following scenario. I have Exchange 2003 in onewindows 2003 AD forest (abc.com in windows 2003 forest functional level) with email address like <users>@xyz.com. Now I setup another AD forest (def.com) on windows 2008 DC (windows 2003 forest functional level) and installed Exchange server 2007. What I want now is exchange 2007 mailbox users are also want to share the same SMTP address of Exchange 2003 i.e. <users>@xyz.com. I want to share the same SMTP address space between Exchange 2003 and Exchange 2007 i.e.<users>@xyz.com . I referred more articles but all are confusing me. I want a straight forward process for doing the following activity, Need to forward mail of unresolved recipients from Exchange 2003 to Exchange 2007 Need to forward mail of unresolved recipients from Exchange 2007 to Exchange 2003 Need to generate NDR for unknown users (users not present in both the exchange server) and which server willgenerate NDRso for above activity what can i need to do on exchange 2003 and exchange 2007... It's very urgent .....please help..... Saravanan

Hi Have you seen below? Exchange 2007 SMTP Namespace Sharing and Different Relay Domain Types http://www.msexchange.org/articles_tutorials/exchange-server-2007/planning-architecture/exchange-2007-smtp-namespace-sharing-different-relay-domain-types.htmlVinod |CCNA|MCSE 2003 +Messaging|MCTS|ITIL V3|

Hi vinod,Thanks for your reply...I am created smtp namespace sharing between exchaneg 2003 and exchaneg 2007 based on the http://support.microsoft.com/kb/321721and http://technet.microsoft.com/en-us/library/bb676395.aspx. I can able to send and recieve mails from both exchange 2003 and exchange 2007 servers for valid users. When i send mail tounknown user from any one of server for e.g. unknown@xyz.com (not present in both server) "A loop was detected error" happened between both servers. How to avoid the loop between exchange servers? How to generate NDR for unresolved recipient's? Please help me......Thanks in advance....Saravanan

Hi Saravanan, As the KB321721 indicated: If you configure the last e-mail system that is in an e-mail system chain to forward unresolved recipients to the incoming e-mail gateway, you will have a messaging loop in which e-mail messages may continuously loop between e-mail servers. Only one e-mail system can be authoritative for a particular SMTP address space. When an e-mail system is non-authoritative for an SMTP address space, the e-mail must eventually be routed to an e-mail system that is authoritative for the SMTP address space. Therefore, if the Exchange 2007 server is the last e-mail system, it should never forward unresolved recipients to Exchange 2003 again which will result in mail loop. In other words, Exchange 2007 server must be configured to be authoritative for xyz.com domain. Nevertheless, when users in Exchange 2007 attempts to deliver message to users in Exchange 2003, you may encounter error indicated the user does not exist because Exchang 2007 is authoritative for xyz.com domain. To resolve the issue, you need to refer to the method 2 in KB321721 to create contact in Exchange 2007 for users in Exchange 2003. ~~~~~~~~~~~~~~~~ Mike Shen TechNet Subscriber Support in forum If you have any feedback on our support, please contact tngfb@microsoft.com ~~~~~~~~~~~~~~~~