i had an experience with mass mailing some time ago with a marketing company i worked with, to avoid to get you black listed the first thing you should do is set a timer to send batches of emails for example 10 emails per second, the mailing list we used at that company had over 50000 emails each list, this way it takes a bit more time but first you wont place your server under a heavy load, and second you have less probability's to get blacklisted.