You don’t have to call it a newsletter, but there are mailing list managers or CRM applications that allow you to do what you want (and probably better than discourse would ever be able to do it). Since these are mostly used for marketing purposes, you need to think (and search) in terms of marketing, sales funnel, marketing campaigns etc. but technically it’s exactly what you want.
I don’t know, though, whether or how it is possible to set up an automatic workflow at user sign-up. Would also be curious to learn about existing setups and experiences.
In fact, it looks like there isn’t any new user webhook yet: