Microsoft Outlook support has often been asked if it is possible to automatically have the “From” field populated with a shared mailbox owner’s email address. This question generally comes from a user who is either a Delegate for that mailbox owner (with added permissions) or a user who has “Send As” permissions.
Typically, you would access the Inbox of the shared folder and then click on the New E-mail button in the ribbon. When the New Message window comes up, the “From” field is populated with the default account owner’s address.
How do you get the “From” field to show up in new e-mail message? It’s easy, simply create a new e-mail message and then click on Options and then click on the “From” field from within the “Show Fields” group. Next, close the message without saving.
The result of this action is that the “From” field will now be exposed on the new e-mail message from now on.
To send mail “As” another user, you would click on the “From” and then choose the user from the GAL:
As you continue to use Outlook to send mail As another mailbox owner, the “From” field keeps a Most Recently Used (MRU) list.