Email lacks an inherent Follow dynamic. Blog + RSS / Atom will give that.
You can use mailing lists, though those impose a bit of additional friction in several dynamics.
There's also the matter of format conventions and client rendering, which can be quite significant particularly across various schools / application models: Usenet / Unix style top quoting, ccmail, LotusMail, or Exchange quote & reply styles.
What you're listing sounds very similar to... email or one of the many instant messaging platforms.