"this post arrived by email"

I’ve seen this on quite a few posts now, and I don’t really understand. How are people posting to a message board by email and why?

Go to your profile >> preference >> email and enable email notifications. That should let you post replies by email.

Kind of a funny sentence since ‘post’ can also mean a physical letter.

To possibly clarify a bit more: Discourse is set up where you can reply to an email notification, and it will show up as a new post.