I disabled the built-in administrator account. I copy it and created my new admin account, let's called it "expert". I tried to assign some domain users to send as "expert", but nothing it's working. The message it's something like "I don't have permission to send on behalf....." But I am not doing behalf anyways...
This is only working when I assign domain user to another domain user for send as. But I don't know why only the admin accoutn not working. Here is what I did:
Log onto the server running Exchange.
Run Active Directory Users and Computers.
Under the "View" menu ensure that "Advanced Features" is ticked.
Find the user's account that you want to be able to send as, and open up the account properties.
Select the "Security" tab.
Click [Add ...] (under "Group or user names") and add the user (users or group) that is to be granted permission to send-as this account.
For each account added, highlight the account under "Group or user names" and in the "Permissions for ..." window grant the account "Send As" permission.
Click [OK] to close the account properties dialog.