Darin is almost 100% correct. The law does allow ONE email, the first one you send unsolicited, any after that if they don't give you permission to send more is considered SPAM. So the first email better knock their socks off. They can still mark it as SPAM, and you will legally be fine; BUT it won't stop your ISP, email provider, program or others from black listing you or cancelling your accounts.
You can run a list from your email account. It is a PITA but possible. It does take a lot of time and when your list gets bigger is almost certain to drive you to drink. As long as you have a "If you don't want to receive emails anymore, please reply and put UNSUBSCRIBE in the email" or something similar in each email. You do run a greater risk of getting your account hit with a SPAM complaint and closed.
There are scripts and plugins for WordPress that are true autoresponders. You can import any addresses you want. Problems arise if your hosting has limits on how many email you can send in a certain time or has a bandwidth limit on the account level you have. If your domain gets blacklisted on the SPAM list, it could be very bad since you have no options to take your list elsewhere.
There are also a glut of "free" autoresponders out there and bulk email programs. Some will let you import lists. These come and go quite frequently (shut down when they get too many SPAM complaints) so you take your chances. The bulk email programs have the limitation your ISP will allow since you have to do it from your desktop email client (usually). There are a few online ones but few are "free" and charge per email to send or you have to buy the software.
Though it sounds like you are getting a list either from a program or buying leads in bulk, neither is ever a good idea since you have no way to know how these leads were actually gotten or if they are even good addresses. Every single time I have done this in the past, even from supposedly reputable places - it ended badly. The emails could have been scraped from sites and you will get into hot water from a deluge of SPAM complaints. So just say no....
It is really not that hard to build a list of your own people. There is a HUGE amount of information for free online to do it. There are a lot of good autoresponders out there. Most have a free 30 day or have to have so many subscribers before they charge (I prefer the number of subscribers as it isn't a quick process to build a good list usually) Then you know where the leads are coming from and are legally protected from SPAM complaints since they did opt in. Well, as long as you use a double opt in that is.
Building a list is like getting married, once you are in an autoresponder you are pretty well stuck with it for the duration of your list. So you need to make sure you can "cover the nut" or the expense of it with your list building or you will go broke pretty quickly before you build it up enough to create and income.
Lastly, don't choose one for the commissions. There was a study I read a few years ago that over the long haul, they pay about the same. You need to make sure you can afford the monthly bill until your list starts paying for itself or just covering your cost each month. You also want to look at what the price increases are for certain levels down the road. You can tell who is making money with their list (or just blowing money in large qualities) by who does their list. Some are several hundred dollars per month even with a small list because of the extra features they provide.
Hope this helps!