1. Check whether your user account is listed in the relevant organizational unit in the Active Directory Users and Computers console.
2. If your user account is not listed in the relevant organizational unit, contact your system administrator and ask them to add you.
3. Make sure your Active Directory user account is in the correct container, and that the correct attributes are set in the Active Directory user account properties.
4. Check that the Active Directory user account is assigned the correct global security group.
5. Assign a valid email address to the Active Directory user account.
6. Validate that the account is active in the Active Directory Users and Computers console.
7. Perform a global address list (GAL) update and reset the Offline Address Book on the Microsoft Exchange Server.