Sending user on behalf of delegating user
In order to get rid of the “on behalf of” in send message and have it so it just appears as being from the delegating user, add the sending user has to have “send as” security permissions in the delegating users active directory profile.
1. Start Active Directory Users and Computers; click Start, point to Programs, point to Administrative Tools, and then click Active Directory Users and Computers.
2. On the View menu, make sure that Advanced Features is selected.
3. Double-click the user that you want to grant send as rights for, and then click the Security tab.
4. Click Add, click the user that you want to give send as rights to, and then check send as under allow in the Permissions area.
4.5 Remove all other permissions granted by default so only the send as permission is granted.
5. Click OK to close the dialog box.