Thank you Phons.
This is a very logical and structured feature. However, it seems to me people will use it only in exceptional circumstances.
Compared to the flexibility of a Send to several CC, you would need a process in which it is absolutely compulsory that everybody replies in that precise order... You should also be certain that no recipient will block the process by not opening his mailbox in due time, or not reading that message in due time...
Maybe in some legal approval processes, like a list of "Read and Approved". Or some hierarchical back-office decision process, the message then becoming a sort of mini-wiki...
And you would also need a situation when all users, without any exception, understand that they are handling a circulation message...
Worth keeping, anyway. But, due to its extremely rare use, shouldn't it be in the Special tab, instead of the top bar menu when you decide to start a new message? It seems to me a "Send attachment", that would lead to a new message but chosing the attachment first, would be at least a hundred times more used than "Send a circulation message".
Or, better, shouldn't it become some choice to make when you add recipients?
EDIT: By the way, I think that, compared to the benefit of starting a message immediately when clicking on New Message, all the items of that sub-menu in File -> New Message could be moved to later stages.