Exchange Management Console
The Exchange Management Console is the admin program for Exchange 2010.
To Grant Full Control to a user's mailbox
This will allow another user to 'Send As' the first user.
- Open the Exchange Management Console
- Expand "Recipient Configuration"
- Right mouse click on "Mailbox" and choose find from the list.
- Find the user you would like to grant permissions for (ie: the mailbox of the user who will have the messages sent at).
- Right mouse click on the user and choose "Manage Full Access Permissions"
- Add the user who will be able to send as the first user to this list.