I get this warning when trying to message them, "User 'stef_stef' has not chosen to allow messages from newbies."
It's very simple that you can not message them if anyone opted in not to receive messages from newbie. You will need Jr. Member rank at-least.
You are right, if a user enabled not to receive emails from newbies in
personal message options, newbies will not be able to send email to the user, but I think the option not to let newbies to be able to send message to higher ranked users is by default, which means someone would have to option in for it to be able to receive emails from newbies. This is what I noticed on my account, I will like your correction if I am wrong.