Yes - user machine refers to the web machine for your site.
Nope - other than getting a dedicated machine. There's currently not an official policy on this (other than the general policies which say "if you're using up too many resources, you can't do this), but we're moving in the direction of not allowing it or severely limiting it.
Define "opt-in", though. Our definition of this may not agree with yours. Have all the recipients confirmed their subscription via a unique token, and do you have the other information we require in our spam policy? If not, you will need to confirm the existing subscribers so that anyone who was maliciously or accidentally added to the list without their consent, or no longer wishes to be subscribed will stop receiving the messages.