I don't think there's a big technical issue in scaling SMTP sends; you can have N machines doing it in parallel and it will work (almost) N times faster than one machine. There IS the question of minimizing your cost per unit email, and the real spammers address that by building SMTP senders that don't comply with the standard.
What does bug me about email is deliverability. It's pretty much impossible to send e-mail to AOL members, for instance, if you (1) don't pay ice to AOL, or (2) are big enough that AOL is afraid you'll sue them. AOL has shrunk a lot lately, so this isn't as big a concern as it was years ago.
It's not hard to get burned by other organizations as well. For instance, I sent an email shot on the the behalf of a campus organization at a university from off campus... an opt-in list of 2500 subscribers, really just chicken feed, but only 1999 went through -- after 2000 connection attempts, they firewalled my IP address and for all I know that address is still firewalled today. I knew the guy who runs email for that school by name and refused to talk about the whole affair... That's what you're really up against.