Comment by jech

>> You could have messaged an entire organization or department

> This is a mailing list.

The way I understand it, the layering is different. In X.400, multicasting was a feature of the protocol. An SMTP mailing list, on the other hand, is an endpoint that terminates a protocol transaction, and then initiates one transaction for each final recipient.

I guess it boils down to where it is preferable to have the extra complexity: the ITU-T protocols invariably prefer to put it inside the network, while the Internet protocols prefer to put it at the endpoints. The SMTP protocol is simple, and therefore the mailing list software needs to be complex.