My definition of a niche is (IMHO) the best I’ve ever heard, as it relates to helping you land clients… A niche is a group of people that are easy to find and contact, who all speak the same business or social jargon, and have a history of hiring copywriters. Can you find a list of people looking for email copywriters? Not likely. Can you find a list of golf companies? Yep.
BTW… In my experience, people that specialize in a given writing style usually struggle financially. Not many companies think in terms of “I need to find an email writer.” They think, “I need to find someone who understands my industry and talk to and persuade my audience.”