It will cost a LOT to send that many by paying someone, so I'd advise against it unless you're gonna be making 1k+ of this, not to mention you'll have to deal with a non-legal business (unless you've managed 1m opt-in's) and if your list is unique... well I know I wouldn't trust anyone with it, if it's not unique then it might not even be worth your time.
Response rates would most likely be less unless your list is fresh/targeted. I'm assuming you've scraped the emails/picked them up somewhere!
Take a look at this if your up for doing it yourself;
http://www.blackhatworld.com/blackh...omains-10-monthly-sending-432-000-emails.html
You could actually outsource the creation of that if you don't have any background in webdev.
Best of luck.