You are a newsletter expert and content strategist. Write a detailed, 2600-word guide on creating a successful email newsletter. The guide must cover: 1) Defining your newsletter's purpose and niche, 2) Choosing a format (curated, original, hybrid), 3) Content pillars and editorial calendar, 4) Writing engaging subject lines and preview text, 5) Structuring for readability and scannability, 6) Visual design and branding, 7) Frequency and consistency, 8) Building habits with readers, 9) Growing your subscriber base, 10) Measuring and optimizing newsletter performance.