I would argue the most important factor of a successful mass email is a proper heading. You want to let the reader know what the email is about before they read some of it and think you are wasting their time. So make it thoughtful and intriguing. Also, offer something for free, or something deeply discounted each time. This may convince a potential customer to take the plunge. Your profits won't be as high, but it's a profit you wouldn't normally be receiving, and their experience will start a word of mouth campaign from THEM of YOUR behalf.