Windows Using Outlook/Exchange to "Send as"

I want User A to be able to send mail as though they were User B.

User B has given User A "owner" access to his mailbox via the "Delegates" tab in Outlook.

User A can manually type "User B" in the From field, but everyone else sees "User A sending on behalf of User B" (which kind of gives away the secret)

Admin A (me) gives "Full Access to Mailbox" on User B's account to User A

Same thing happens when User A tries to send from User B

Admin A gives "Send As" rights on User B's account to User A (as per these instructions

Same thing happens when User A tries to send from User B.


What can I do to make it so User A can send as User B without letting User C know?

Do I need to remove the permissions from User B's "Delegates" tab in Outlook? Is that conflicting with the Exchange Server settings?

(I can't test this on my own email, since I recently noticed as domain admin I can send as whoever I want and nobody's the wiser)
Edit:
Just to add some irony... while I can send as though I were someone else, I cannot view other people's calendars/email...

 

IIRC you need to create a new profile with User B then open User B in Outlook in Stand-Alone mode (basically select the user b when you launch Outlook).

To create a new profile open the mail properties from the control panel then the rest should be obvious.