It is spam wither way you look.
Mailchimp allows you with imported lists, but for 1M you will have to pay few thousand of dollars to import them. Unless you import 2k, send, then import another 2k, but as you say you need to sent a lot and this might not be an option. Anyway, you will be banned very soon due to high bounce and report rate.
You can also send it from a server, but you need to send less than 500 per ip per day to not get into spam folder quickly. This assuming that the IP addresses are not black listed and you send the email right so it won't trigger spam filters.
Anyway, this does not seems to be a viable business model.