Postmark is a developer favorite due to its easy integration, reliable delivery, and comprehensive feature set. It offers a volume-based pricing model with no feature gates or paywalls. In contrast, SendGrid is an older player in the email API space that provides a broad range of services beyond just an API, but has received mixed reviews regarding customer service quality. SparkPost is a popular choice for Enterprise customers due to its focus on security and compliance, while Mailgun's API is powerful and flexible but lacks official libraries in some languages. Amazon SES is the cheapest email API service with limited features and a stripped-down service, making it less suitable for most use cases.