I would unsubscribe for that list immediately mailing list and maybe report them for spam. A second, and primary factor, is ensuring the source of that list. Where did it come from, and what EXACTLY did they sign up for in the first place? Personally I don't believe in buying lists, you can always ensure the list is sound when you have built it yourself.
Although it is fair to say a Joint Venture with a partner in the same Niche is a sure bet and in this case you would have their permission to mailing list send their list an e-mail regarding a product or service that they had subscribed for. A third and important factor is called the double opt-in. Now this has been around for a while and I know a great deal of people have suggested it is a killer on e-mail marketing.
I say this to them - rubbish. If you have a product or information that is well worth their time, the Right kind of customers will not mind at all mailing list clicking on an e-mail to verify their subscription in order to get that product or information. In most cases REAL customers like that added level protection. Now I use the term "Real" and "Right.