There is nothing "self absorbing"  about believing an intra-site mail system should work in a sensible way, and the fact that it is even possible to put what are supposed to be private communications on a public timeline, defies traditional logic.  It's akin to sending a traditional email, with an option to CC everyone on the internet,  While there may be  a few people that think it's a great idea, the majority of the planet's population will be thinking "Why would I ever want to do that."

I'm sure Alex T⚜️ might have a good explanation for it, and I'm anxious to hear it.  I do not care that it can be turned off... I'm curious as to why it's there in the first place.

Another thing I find odd about the "private inbox for asynchronous on-site  communication between users", is that there is no 'Subject' field.  Do you find that odd?