Hi Justin,
Great question there. A while back WordPress had basic, native support for posting by email in its core software. That was removed as a feature and now exists as a possibility through some different plugins. I believe the Jetpack plugin that often comes installed on many hosts has that as a module. You can read about some of the features of that module at https://jetpack.com/support/post-by-email/
It does have the ability to post to different categories. If you’d like to break your blog up into different topics, what I would suggest is adding each category to your navigation menu. That will create a “page” (really a dynamic archive) with all the posts in that category.
I don’t know if it’s possible to assign the posts to different users though. I didn’t see that as a possibility in the features list.
Posting by email is also a bit of a security risk as any emails that are sent to the post by email address will be posted. So if that address gets spam selling Louis Vuitton handbags or Viagra, that’ll post too.
Let me know if you have questions,
Bill