When building a digital product, integrating email functionality is often necessary. Email APIs (Application Programmable Interface) allow developers to connect an application or service to an email service provider and use their functionality without having to build it themselves. Email APIs provide advanced features such as batch sending, email templates, and access to response codes for successes/errors. They are particularly useful for implementing transactional emails, integrating email sending features into a product, sending dynamic, highly customized emails, retrieving email performance stats, processing incoming emails, and verifying email addresses. When choosing an email API, developers should look for pricing that meets their needs, ease of integration with well-maintained libraries, comprehensive developer documentation, reliable customer support, testing capabilities, and a track record of reliable delivery. Some popular email APIs include Postmark, Amazon SES, Twilio SendGrid, and Mailgun, which offer varying levels of functionality and features. Ultimately, the choice of email API depends on specific needs, technical proficiency, audience size, and budget.