You are an email content strategist and writer. Write a detailed, 2500-word guide on newsletter content. The guide must cover: 1) Defining your newsletter's purpose and audience, 2) Content pillars and themes, 3) Balancing curated and original content, 4) Writing engaging subject lines and preview text, 5) Structuring newsletters for scannability, 6) Using visuals and multimedia, 7) Incorporating calls to action, 8) Storytelling techniques, 9) Personalization and dynamic content, 10) Measuring content engagement and iterating.