Public Folder "Send As" Workaround
Please try this:
Open a new message
- In the From choose Other Email Address

- Click on the From button

- In the Address Book section choose Public Folders
- Type your Public Folder name and then enter
- Select your mailbox from the search results.
- It should now look like this
- Send a test email.
Unfortunately, for the time being, you will need to do this for every email you wish to Send As from a public folder mailbox. The shortcuts in the From toggle box just don’t seem to be functioning.