I never said there are no reasons to use such a system, but anyway thanks for the lengthy elaboration of osrf’s reasoning.
I didn’t know that short thread got so much attention, but it kind of makes sense. It actually made me laugh when someone suggested the user list got too active.
Especially as it feels like every mail that is sent by someone other than yourself gets a fast reply saying “go to answers.ros.org”, hehe.
I’m pretty sure one day of subscription to the Linux Kernel Mailing List would change their mind (and their way of handling mails). 
You should be able to send a message to ros+[categoryname]@discoursemail.com if you are at trust level 1 or higher.
From your reply on github you implied that the domain did not resolve for you.
I forgot to look up their MX entries(actually just one). I simply weighted it more likely that you made a typo
than that all mails sent for any discourse instance are send over a central mail server.
Still sounds somewhat fishy to me.
So I suppose it works as you described and I’ll test it eventually. Thanks!
In terms of filtering, mailing list mode means that you get everything on the forum sent to you, just like a mailing list would.
I enabled mailing list mode only recently because I didn’t get a number of replies from Migration to one GitHub repo for MoveIt! - #21 by 130s .
Although I received some other posts from the watched category…
Looking into it a bit, I suspect that discourse did not automatically send mails to me for replies to the thread because the thread got created before I added the category to the watch list (and if I remember correctly also before I created the accout).
I’m not sure this is the case, but it might actually be a bug in the system?
I turned off mailing list mode again and I expect that everything works as expected now.
If I notice any further problems, I’ll come back to this.
Thanks for your time.