I generally have grief getting send as to work, one of the main reasons being that I set the correct settings but it takes a while for them to update.
Here’s the instructions for setting the user permissions
- From Active Directory Users and Computers go to the “View” menu and select “Advanced Features”
- 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 …] and add the user group that wants to be able to send-as.
- 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.
Outlook reads a lot of security information from its address book and not directly from Exchange. So if after you have followed this procedure you find that Outlook doesn’t let you send as another then you need to force an update of the address book in system manager and then on the users outlook client.
Here’s a couple of other articles on the topic