I agree with everyone - not a single user in our organization enjoys this or needs this. Why add email features in a messaging platform? We already have multiple heading formatting options literally at the top of the message block, why force a subject line on top of that? The 'Start a post' menu just shouldn't start in formatting mode, period. If users want to format email-like messages when posting, let them press the format button, don't force it for every new post.
What a messaging client workflow should look like:
click > type > enter
What it currently looks like:
click > tab > tab > type > ctrl+enter
See the problem here? K.I.S.S.