All, I have just setup a new Exchange 2003 sp1 server in the UK. I am migrating the users from the existing 5.5 sp4 server within the UK site to the new 2k3 server. The ADC box is in the US, at my site. All of my Exchange servers are in the same site. Four Exchange 5.5 servers and one Exchange 2k3 server. When I move users in the UK to the newer server, and give it some time to replicate, there are no problems in sending or recieving emails from those users. Both to and from the Exchange 5.5 or Exchange 2k3 users. When I create a new user ID with "AD Users and Computers" and go through the same to create the email account when prompted, it will of course only let me create the new account on the new Exchange 2k3 server called "Jaguar". This is appropriate I would think. But even though I can see the email account on the 2k3 server, and I am able to log in as this let's say test account at this point, connect to the email server with the Outlook client, it resolves the name of the server and email account just fine, and lets me send email to users both on the new 2k3 server and my existing Exchange 5.5 servers with NO problem. But when I try to send an email to this "AD Created" user from a 5.5 account, I get an "Unknown recipient" NDR back on the 5.5 client. If I create another "New" account, using the "AD Users and Computers" to create the ID only, and create the "New" email account using the Exchange 5.5 manager, and then move the mailbox to the new Exchange 2003 server, wait for a little replication time, the emailbox\client works just fine in getting or sending emails to and from any of the users, be they Exchange 2k3 or 5.5.. I tend to think this to be an "ADC" issue in replicating the "Exchange 2k3" users to the "Exchange 5.5" address list. It seems that if the "user" is or has been in the "GAL" on 5.5, there is no problem in identifying the user or his server, but if the "user" has never been in the "5.5 GAL", then the ADC is not populating it from the "Exchange 2k3 GAL" to the "5.5 GAL". Does this make sense, and does anyone have any clues as to how to get this functional. HELP!! Thanx in advance for any help Bob Hall POOL CORP.