I’ve set up Discourse so that the welcome messages come from my user – and I’ve made them a little more personal to match. 
I note, though, that all of these messages are considered “unread” for me. Would it not make more sense for them to be marked as read by default, unless and until someone replies?
Strangely, they don’t show up in my “sent” folder – but they do show up in “Suggested messages” when I’m reading other messages.
(I’m not convinced by the utility of “suggested messages”, but that’s a discussion for another time!)