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.
 * 1) Open the Exchange Management Console
 * 2) Expand "Recipient Configuration"
 * 3) Right mouse click on "Mailbox" and choose find from the list.
 * 4) Find the user you would like to grant permissions for (ie: the mailbox of the user who will have the messages sent at).
 * 5) Right mouse click on the user and choose "Manage Full Access Permissions"
 * 6) Add the user who will be able to send as the first user to this list.