Some of our users who through their personal email (hotmail / gmail) with IE7 sent mail to their business address (on the exchange in 2003) regularly get a pop-up username and password prompted. I have not been able to reproduce it myself, but many users complain about their customers that receive this pop-up message when their trying to sent to someone on our exchange server.
Some of the local users in the LAN who are using Outlook 2003 clients also complains that they having this pop-up issue regulary. Some of them use Windows XP en some Windows Vista. The SBS2003 server is up2date and the latest SP2 is installed for Exchange 2003.