A quick tip for anyone considering an email marketing campaign: Do your homework on the list you're considering.
I'll back-up. There are two types of opt-in email marketing lists: addresses you gather on your website and those that you buy from a vendor.
An example of a Wheel Media client website with the former is www.pcpipe.com. This is a very reliable way to gather a list of customers and prospects. The list will grow slowly, but the quality of the list is high because people have pro-actively taken action. An excellent strategy.
Buying a list is a bit more tricky. You'll need to confirm that the names are opt-in, that you aren't paying for duplicate email addresses, and that if there is a problem with the list they will work with you. The most reliable way to conduct a mail campaign is to use an agency for the content, design and coding, and a dedicated service for the mailing. This ensures you comply with spam laws.
This mailing service also will also do a cursory review of the list to ensure it's clean. This review checks for fake email addresses (i.e., fdsfsdf@sfsdf.com) or a high percentage of globally-unsubscribed email addresses. If it does, you won't be able to send to it, period. (And you wouldn't want to, because emailing people that don't want to hear from you does more harm than good.)
More info here.