You should use a bulk email marketing service to send that many emails on a list simply because a regular email account would be blacklisted and marked as a spammer when exceeding 50 emails in a single email.
What You Should Be Looking For...
You want a bulk email service with a high delivery rate, where your emails won't end up in a spam filter of the people you're trying to reach. The email service should (of course) have the ability to import lists from the start - some services do not.
What Services Are Recommended...
All of the following services allow you to import bulk email lists.
MailChimp - Has extremely high deliverability and incredible API customizability. You can send emails for free forever though you only get auto-responders when you pay. This one's probably the very best you can get.
I personally prefer the design of MailChimp over all other recommendations.
Constant Contact - They've been around a long time and are pretty solid. They cost the same as Aweber (which you can't use for bulk imports) and have amazing customer support. They can easily handle lists of 30,000 or more.
They also provide a 60 day free trial.
MyBizMailer - They have good delivery rates and give you a free trial. I haven't personally tried this one though I hear it's okay relative to the first two.
FeedBlitz - This email service is one I've tried before and they definitely do bulk imports. They're pretty decent and Phil is on Twitter and gets back to you asap.
Is It Worth the Cost...
If you really want to reach those customers then it's worth the cost. MailChimp will cost more if you go on paid subscription however its high open rates are a match for Constant Contact. If you desire flexibility and integration in the future go with MailChimp.
It also has a slicker design.
I find Constant Contact to be solid yet when I last looked at them they had less of the social media, RSS bells and whistles that MailChimp and FeedBlitz have. Worth the cost? That depends on what else you're doing.
What to Keep in Mind...
If you're buying an email list you're going to run into trouble with any kind of bulk email service because the bounce rate may be so high they shut down your list. This is why email services like Aweber use double opt in to make sure everyone on the list actually wants to hear from you.
MailChimp for example will close your account if your bounce rate is 5%+ on imported leads so I've heard. It's likely other services may have similar restrictions.