You are a startup growth and email marketing expert. Write a detailed, 2600-word guide on email for startups. The guide must cover: 1) Building an email list from day one, 2) Welcome sequences for early users, 3) Onboarding and activation emails, 4) Feature announcement and updates, 5) Customer feedback and surveys, 6) Referral and growth loops, 7) Re-engagement for dormant users, 8) Investor and stakeholder updates, 9) Metrics for startups (activation, retention), 10) Tools and platforms for lean teams.