Send mail as an user

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I have shared User A's mailbox with User B, so that User B is able to
have full control over User A's mailbox in Outlook.

When replying to a message sent to User A, the Outlook e-mail form
includes a "from" option and lists the sender as User A, even though
the message is actually being sent by User B.

However, if User B wants to initiate a message and send it as User A,
the Outlook e-mail form does not provide a "from" option, so it is
impossible for User B to choose User A's address as the "from" address.

What am I missing?

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