Another type of electronic mailing list is a discussion list, in which any subscriber may post. On a discussion list, a subscriber uses the mailing list to send messages to all the other subscribers, who may answer in similar fashion. Thus, actual discussion and information exchanges can happen. Mailing lists of this type are usually topic-oriented (for example, politics, scientific discussion, health problems, joke contests), and the topic may range from extremely narrow to "whatever you think could interest us". In this they are similar to Usenet newsgroups, another form of discussion group that may have an aversion to off-topic messages.
If you say, “I want to see what type of response I get from my first email before I send more”, it’s not going to work. Email marketing is marketing, not magic. You wouldn’t expect to get a great return on playing a radio ad or running a TV spot just once.  Don’t expect a great return from sending just one email. Tell yourself you’re going to test the effectiveness of email marketing for a period of time (a full year is a reasonable amount of time). Remember that customers will open emails differently depending on the time of year and even the time of day that your email is sent. To get as many customers as possible to open your emails, you need to commit to sending emails over a period of time.

I think what Brian and the testers are missing is that 15k is neither big enough to be impressive nor small enough to be inviting. It’s not a number that works effectively as social proof, and while I can’t test it out myself, I believe, based on evidence for social proof around the web, that DIYthemes would have had much better success with a combined number in the 50k+ range as mentioned above.

It's practically impossible to overstate the importance of direct mailing lists to the success of your direct mail program. The correct mailing list will contain your most valuable prospects. The more careful you are in analyzing and selecting direct mailing lists, the better your chances for success. There are several different categories of mailing lists available on the market today ranging in cost and appropriateness for your market. When you are considering what type of mailing list to buy consider the following three types: