If you're building an application that sends emails to confirm new accounts, reset user passwords, or handle billing and invoicing, then it's sending transactional emails, a common functionality that many vendors provide. To choose the right transactional email API for your application, look for a service with features such as standard email authentication methods, email testing, shared, bring-your-own, and dedicated IPs, integration with other AWS components, and analytics capabilities. Pricing should be considered based on the cheapest plan and its limits, as well as how the pricing scales. Among the top 6 transactional email APIs recommended are Amazon SES, Mailchimp Transactional, Mailgun, Postmark, SendGrid, and SparkPost, each offering unique features, deliverability strategies, and pricing structures that cater to different needs and volumes. Choosing the right service can be a complex task, but considering factors such as speed, security, ease of use, analytics capabilities, and pricing is essential for making an informed decision.