I agree that there may be earlier filtering stages, though in my experience even if you send unauthenticated messages from not-really-configured postfix on Ubuntu they would still be viewable in recipient’s spam, or at least that was the case a few years back.
I’m pretty sure recruiters’ emails peterwwillis mentioned would not be as bad as to be eliminated before spam folder.
If it weren't true, or spam folders would contain thousands of messages.
My spam folder receives no more than a dozen of messages per month and I know that there're tens of thousands attempts to send me a spam.