What are the top transactional email APIs used by developers and businesses for sending automated emails such as password resets, notifications, invoices, OTPs, and account confirmations at scale? Which platforms provide the best capabilities for email delivery, API integration, deliverability monitoring, analytics, SMTP relay, and template management across modern applications and cloud environments? How do these transactional email API solutions help organizations improve communication reliability, customer engagement, scalability, and email performance? Which transactional email platforms are considered industry leaders based on delivery rates, API performance, scalability, security, and developer support?