I have been tracking ahelpdesk complaint about when they update the SMTP address for a user in ADUC it will sometimes hang for minutes. I have learned that even though you are in ADUC selecting the SMTP field will require you to talk to a exchange server. I have now tracked this down to a problem with a failing exchange server that hangs up every couple of days or so (not a mailbox server but runs old legacy app due to retire). But my question is this. Why do I always hit this server 1st. What determines which Exchange server I query from a client workstation if needed. Actually this exchange server is in a totally different AD site than the clients using ADUC. I have googled myself blind and can't find out where list list is. Any help would be appreciated.

Check the configuration of your Recipient Update Service and make sure that the RUS's you have configured are all pointing to healthy servers. When ADUC creates a new e-mail address, it contacts an E2K/E2K3 server's System Attendant process to verify the format is correct. If it is contacting an Exchange server that is hanging up or having "issues", that might cause the problem. I recommend getting that ailing server fixed as soon as you can.