iCal failing to sync calendars with an Exchange 2010-based server - 'ical needs a valid email address' iCal (Version 5.0.1 (1547.4) - Mac OS X 10.7.2) is failing to sync calendar entries for an Exchange account setup via System Preferences.

While Mail and Address Book are able to send mail or sync to the server, the calendars show an exclamation mark next to the account name, which when clicked brings up the following dialogue sheet:


  "ical needs a valid email address to connect to the exchange account"


Entering the email address of the account has no effect, and clicking the exclamation mark again simply re-opens the dialogue sheet.
Have your Exchange Administrator check to make sure your account is on the GAL (Global Address List). There is an option to hide accounts that might be selected.