Ahh, I was hopeful when I first heard about Mail Drop, that it would work regardless of mail provider because of the OS. Seems I'm wrong. Mail Drop works with Apple emails only, yes?
EDIT: From the Yosimite page "When you’re signed in to iCloud, Mail Drop lets you send large files like videos, presentations, even a folder of images without having to worry about your service provider’s limitations." Sounds like, we CAN use Mail Drop from any service provider!