Yep, I agree. Since it's fundamental to the running of a website, it should be core. If somebody does not want to use a local mail service, they she/he can use a 3rd-party service by installing a plugin.
I really pushed the issue for a local smtp mailer being in the core, but was rejected for whatever reason. It shouldn't be a plugin at all. Out of all of the forums I've used, the local mailer was always built in the core for ease of use and keeps things simple. Can someone explain to me further what advantage making the local mailer would be? I don't seem to understand.
Funny thing is, all this time I've been using NodeBB, I STILL have no mail server connected with it because I'm confused here. From core to plugin, and now we have this... :|