I have access to a mailbox of our company which is only intended to receive feedback from external customers and not for internal usage. The mailbox was therefore hidden from the Global Address List.

I also have Send As permissions for this mailbox as I need to reply to these incoming emails but not with the From name of my own mailbox.

However, each time that I try to send out on behalf of this mailbox, I get an undeliverable message stating that I don’t have permission to do so, even though I do!

How can I send out emails on behalf of this hidden mailbox?