SendGrid (by Twilio)

SendGrid, a Twilio company, is a cloud-based email service platform renowned for its powerful API and transactional email reliability. It serves both marketing emails and critical system notifications (password resets, receipts). Its strength lies in superior deliverability infrastructure, detailed analytics, and tools for developers. While it offers a marketing campaigns interface, it's best suited for tech teams and businesses that need to programmatically send high volumes of email with maximum reliability.

Its free tier is generous for testing and low-volume sending.