consider this case:
user A sends an email to admin, (allowing him to inject his email to the forum)
admin forwards the email to the forum,
a topic is made in the forum with “admin’s email”, while the topic owner should be “user A” email.
and since user A is not created in the forum, admin can’t even change the owner of the topic to the real owner.
I remember it used to be the other way creating a user based on the original email sender, shouldn’t it work the same now?