Hi, We have Exchange 2007 coexisting with Exchnage 2003.We have created some contacts in our AD with external email addresses (these contacts are hidden from Exchange Address List).When a Exchange 2007 user sends mail to any of such contact using Outlook, below NDR message is generated:-------------------------------------------------- Delivery has failed to these recipients or distribution lists: There is a problem with the recipient's e-mail system. More than one user has this e-mail address. The recipient's system administrator will have to fix this. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator and then try resending the message after the problem has been resolvedSent by Microsoft Exchange Server 2007 Diagnostic information for administrators: Generating server: my-hub-transport-server.mydomain.comexternalcontactemaildrress@otherdomain.com#550 5.1.4 RESOLVER.ADR.Ambiguous; ambiguous address ## ----------------------------------------------------But, when I try to send email using OWA, it succeeds. I do not get any NDRsAlso, if i remove "Hide from Exchange Address List" check mark from AD properties of the contact, I succeed in sending emil through Outlook as well.This issue is being faced by Exchange 2007 users only. Exchange 2003 users do not face any such issue.Please help me.Regards,AnshuAnshu

NDR: 550 5.1.4 RESOLVER.ADR.Ambiguous; ambiguous address Possible Causes: The error code indicates a duplicate address condition in which two objects have the same proxy address in the destination directory Troubleshooting: 1. Please use the method in this article to check if theres duplicate object in the AD 2. Please delete one problematic contact and see if we can recreate it via EMC without error 3. Please check if theres a AD replication issue in the environment, which can cause same symptom Run Repadmin /showrepl, which can verify if the AD replication is ok Resources: Force replication over a connection Similar case: 550 5.1.4 RESOLVER.ADR.Ambiguous; ambiguous address Similar case: NDR 550 5.1.4 in Exchange 2007