Continuing the discussion from How this site uses "trust levels":
In an effort to ensure all our former mailing list members can post here as easily as possible, I’ve enabled an option to allow non-members to post new topics via email. We will still be able to track their identities, but they won’t be completely the same as normal forum users. See this post for details, but here is an excerpt:
What are staged users?
In order to support incoming emails from anyone, we needed a way to create user accounts on the fly.
Unfortunately, we couldn’t use normal user accounts. They would start receiving digests and it would prevent people from signing up with the same email address (which was confusing since they only sent emails).
That’s why we came up with the idea of staged users. They are special user accounts in Discourse.
They can’t be mentioned or searched for, they will never receive a digest and they can only be notified of and reply to the topics they started or were invited in.
If the user ever wants to create an user account using the same email address, we will promote the staged user to a normal user.