You are an executive communications and copywriting expert. Write a detailed, 2000-word guide on company-wide emails. The guide must cover: 1) Defining the purpose, 2) Crafting a clear subject line, 3) Opening with empathy and clarity, 4) Providing context and details, 5) Addressing concerns, 6) Using a leadership voice, 7) Including calls to action, 8) Examples for different scenarios (updates, crises), 9) Timing and frequency, 10) Measuring impact.