I agree with the other answers, opt in is opt in for a specific purpose. You need to be very careful about what and how you send...it can be a very expensive mistake if you don't follow all CANN-SPAM regs.
But to answer your email question, there are hosted STMP relays that you can purchase access to such as SocketLabs and SMTP.com. I prefer these to a MailChimp if I'm going to be doing something proprietary and repetitively. I've had great success re-selling email marketing via Interspire's Email marketer combined with either SocketLabs or my own Co-Lo'ed server doing the routing.
Throttling (which is what you describe) is something my company uses to ensure the big three (AOL, Gmail, MSN/Hotmail) don't peg you as a spammer right off the bat...it could be 40% of your list, believe it or not. I would refrain from using it to "trick" the server hosts, as they'll eventually catch on and the result usually isn't very much fun.