Alright so I've unhidden the shared mailbox, and if I open a new email, click From then Other Email Addresses and select the Online GAL, search for the shared mailbox, and select it that way, the email goes through.

This saves an entry for that shared mailbox in the From drop down, but selecting that causes it to fail again. So the user currently needs to do that whole process for every email coming from this shared box.

I've downloaded a current address book, and set the default to use the online GAL, no difference. So I have a sort of functional workaround, but still not ideal, if you have any other ideas I'm open to anything at this point .