Below are the things you need to do to troubleshoot:

1. Check if the account is active in your Active Directory.
2. Verify if the account is added to the required mail client.
3. Check your Active Directory permissions.
4. Confirm if the user has the correct privileges to access the mail client.
5. Verify if the user is added to the correct client group.
6. Check if the user’s account has been configured properly.
7. Review the past logs to look for any errors.
8. Try reinstalling the mail client.