Your problem won't be sending speed. You're problem will be how much a MTA is willing to accept from your MTA without throttling you and you IP/domain reputation.
Answer the following:
How big is your list?
Is it purchased? If so, have the leads been validated?
# of variables per lead? (name, IP Address, sign-up date/time, etc.)
Your budget? (i.e. what you are willing to pay monthly not including your server costs)
You want to send fast and inbox? It's going to cost you.