Hi there, Environment before Co-existance: Exchange 2003 is sharing the single SMTP namespace (lets assume Domain-a.com) with our Postfix mail server. Postfix mail server is our mail gateway which routes all the mails belonging to Exchange mailboxes to Exchange 2003 FE server. On ESM, we have configured "Forward all mail with unresolved recipient to host" pointing to Postfix server. Co-Existance 1: Installed Exchange 2010 as per best practices. Interop routing group connectors were created automatically to Exchange 2003. In Exchange 2010 is by default, domain-a is configured as Authoritative. Still the mails from Postfix are routed to Exchange 2003. Did testing as below: Mail To Ex2003 To Ex2010 To Postfix To Internet Sent from Ex2003 yes Yes yes yes Sent from Ex2010 yes Yes NO yes Sent from Postfix yes Yes Sent from Internet yes Yes Mails to Postfix mailboxes from Exchange 2010 is bouncing back with NDR as: #550 5.1.1 RESOLVER.ADR.RecipNotFound; not found ## Tried to change on Exchange 2010 server as Internal Relay Domain for Domain-a.com, now the situation as below: Mail To Ex2003 To Ex2010 To Postfix To Internet Sent from Ex2003 Queued Yes yes yes Sent from Ex2010 Queued Yes yes yes Sent from Postfix Queued Yes Sent from Internet Queued Yes In Exchange 2003 server “Messages Queued for Deferred delivery” is building up which means a mail routing loop. Co-Existence 2: Switched the mailflow to Exchange 2010. Now the postfix server is delivering the mails to Exchange 2010. But still we are seeing same problem as described above. Now my question: Is there a possibility to resolve mail looping issue in Exchange 2003 during the co-existence period. Since I have large number of mailboxes in Exchange 2003 I cannot migrate all the mailboxes during a single maintenance period. Pls let me know if I am not clear in my description of the problem. -Aj