Yes, they could stop sending email full stop. There's a reason we wanted Domain Keys. Having the spamming fuckwits just moving from one starry-eyed mass email startup to the next every time they get canned wasn't it.
It's not "spam" if people double opt in, want to receive it, read it, and send nice testimonials about how much they enjoy receiving it. At least, no more "spam" than stuff you order on Amazon is "junk mail."