SBS Migration Issue

A few months ago, I migrated from Small Business Server 4.5 to SBS 2003 on a new server. All users now use the new Exchange server. Yesterday, I took the old server off the network. Outlook pulled up the email from the Exchange database immediately, but was virtually unuseable when attempting to create email. it took three minutes to enter email addresses and ten minutes for new internal mail to arrive. Outlook appears to be hung, but I believe it is just unbelievably slow. The problem was resolved when I put the old server back online. I double checked the Outlook settings and they definitely point to the new server. Any suggestions would be greatly appreciated.