This is a great topic for a defined audience! You have received lots of good suggestions from these other experts who are responding to your question. What I might add is figure out who are top bloggers in your topic niche and contact them to see if they accept guest posts. If so, write an original post for them with a link in your bio that directs readers to you, ideally to a landing page on your own site that provides an opt-in opportunity. Connecting with influencers is one of the many ways to build your newsletter subscriber list. Best of luck, and I would be delighted to help you further if you wish to contact me here on Clarity.