Public Folders do not allow you to send from their e-mail address by default. Therefore, you must make sure that setting is requested before you can send from the alternative e-mail address. Please contact IT Support for more information.
A small window will appear. Click the From... button.
You may be tempted to manually enter in the new "From" e-mail address into this section. You will receive an error message explaining that you don't have permissions to send from this account. You must select the alternative e-mail address from the address book.
If you select an e-mail address for which you do not have "Send As" permissions, then you will still be able to select it, compose the message, and apparently send from. However, you will receive an error reply detailing your lack of permissions.