mail and ical randomly change internal server settings for exchange account

Ever since upgrading to Lion, I randomly find that my exchange mail account, my exchange calendar account and my exchange address book go offline. When I look at the settings, I find that the internal server settings have been changed from the correct address to the word "sites". Cannot figure this one out. I have deleted and added the accounts numerous times. Cannot find any error messages.

Just wondering if anyone else has every experienced this.

Debbie