You are a beauty industry and email marketing expert. Write a detailed, 2500-word guide on email for salons and spas. The guide must cover: 1) Building an email list (in-store, online booking), 2) Welcome sequences for new clients, 3) Appointment confirmations and reminders, 4) Promotional emails for services and products, 5) Seasonal and holiday campaigns, 6) Loyalty and rewards program emails, 7) Referral and friend campaigns, 8) Client appreciation and birthday offers, 9) Re-engagement for inactive clients, 10) Metrics for salons (bookings, retail sales).