File request not sent... no "Share" button

I have this very same issue. I tried all the usual tricks: clearing internet cache, different browser, incognito mode. Nothing will allow me to send an email File Request - to an email address that Dropbox does not recognize. When I entered the email address tied to my Dropbox account, the Share button miraculously works. As long as I send a File Request to someone whom I've already shared files with, it works. If I try to send a File Request to an email address that is not in some mystery contacts file, the Share button is greyed out. This doesn't make sense. I want to send a File Request to someone with whom I've never shared files with and likely won't ever again. If I wanted files from someone I already share files with, I'd just have that person upload the file to our shared folder. At least I can copy the link and email it to the person from whom I need the files.