You are an e-commerce email marketing expert. Write a detailed, 2400-word guide on welcome triggers. The guide must cover: 1) Signup triggers, 2) First purchase triggers, 3) Email engagement triggers, 4) Setting up welcome sequences, 5) Personalization based on behavior, 6) Examples of e-commerce welcome triggers, 7) Measuring welcome campaign effectiveness, 8) A/B testing triggers, 9) Continuous optimization, 10) Integration with other triggers.