Firstly lets be clear about what is an opt-in list. Every business keeps a list of its customers and potential customers. When the list is only made up of people who have willingly joined it by subscribing or registering, knowing you will be contacting them, then you have an opt-in list. On the Internet it is much easier and cheaper to create and maintain an opt-in list.

Note that people generally only join knowing that you will be contacting them with information relating to a 'certain topic'* - we'll talk about this in a moment. Buying lists or copying from other sources goes against the notion of opt-in and sending emails not related to the topics the subscribers originally signed up for will destroy your credibility.

* Think about it, would you join some mailing list that is about a broad range of subjects knowing that you are going to get emails that are totally irrelevant to your interests? Would you have any more respect for the list owner who seems to be the master of nothing in particular and intends to send you mailings about anything he likes (because he doesn't know/care what