Use @ mention for outgoing email
Please add the possibility that users are able to post a conversation or a reply to people or distribution groups, that are OUTSIDE of the team. In other words, it's an outgoing chat that is delivered by Email. This can be done by using the @-letter followed by a full Email-Address.
People outside the team that get this Email could reply via Email as well because the sender address is the Team-Channel-Email-address.
This feature will enhance Interoperability with Messaging heavily.
Additionally the post could be flagged with a fitting "outgoing-"banner or text something else, to show Teams user, that this post is not only inside the Team.