I imagine that as soon as you posted in the thread, you would be subscribed to it and, thus, get all posts made in the thread via email.
And in my sites case, STILL have to go back and visit the site to actually see the content... all you get is a notice - it doesn't send the contents, and that was by design... to encourage the user to return to the site and mayhap find additional content they are interested in.
Works well on the Discourse platform.
It may "work well" for the limited interaction it allows... but once more... it's VERY limited in it's functionality, it requires additional bandwidth for mail, and unless you are using a paid mail delivery service, the return/notification email delivery may be spotty depending on the recipients mail provider (think HotMail and how it likes to eat emails) and how clean your senders reputation is.
And once more, I don't see where actively encouraging users to NOT visit your site is of much benefit. With the way XF works on a mobile device, you don't gain THAT much by "reply by email" in time savings.... unless your user specifically does NOT want to have to visit for some weird reason, like trying to avoid getting caught browsing on their mobile device at work.
And if they are going to be spending minutes scrolling down their incoming email list and checking each email from a busy thread and then possibly replying to it.. how exactly does that save time when compared to logging onto the website and doing the same thing? I can see it being an edge case use, but not much more than that.
Honestly... there are many other areas of improvement/advancement I'd rather see pursued.