I just discovered that, Office365 let’s you send messages only, if it is your own domain.
If another institute is organizing a meeting, they can’t use their mail for sending messages to these members.
As a wish I think it would be nice, if everybody within indico could send the message from a fixed internal address but within the message add “reply-to” and that is then the mail address from the user.
reply-to: @ (or if it’s about a meeting, let them choose from which managers mail address)
That would avoid problems with exchange online (Office365).