I'm pretty careful when i'm doing mass mailing and I usually advise a clients to do mass mailing through a third party company that does mass mailing for a living.
1. You can't be to careful with your domain. Mass mailing from your server or domain can cause issues with people marking you as spam. You could run the risk of your ISP banning you or worst having your domain marked as spam by larger providers.
2. Third party solutions have better tracking of your mailing campaign.
3. Following CAN-SPAM laws are very very important. 3rd party companies usually force you to comply.
Joomlashack.com as something that should work for you if you want to use a third party solution.
you can read more about it at this link. This is a pretty nice written API to icontact.com
http://www.joomlashack.com/index.php?option=com_content&task=view&id=195
Good luck !