It could be Business, Humor or Travel, but nevertheless they are reading your words for a reason. Your job is to supply your readers with the information they are looking for. This is not a venture to be entered into lightly. Newsletters are not overnight successes. It sometimes takes years to build a respectable subscriber base.
Cash flow is almost non-existent at the beginning and for much of the time after that.
You must earn, gain and keep a certain amount of trust and loyalty from your readers. Running a newsletter is much like being in a political office. You would not be there if you did not serve your constituents to their satisfaction. They will directly or indirectly dictate to you your requirements in serving them. If you fail so does your newsletter.