SparkPost

Budget-friendly email API by Netcore Cloud, popular with high-volume senders

Email sandbox for testing, paired with a production transactional sending API

Deliverability-focused transactional email infrastructure for developers

Budget email API combining transactional sending and marketing campaigns

Reliable SMTP relay and email API with a permanent free tier

Mailchimp's transactional email add-on API

Transactional email API from the MailerLite team, with generous free-tier limits

All-in-one marketing platform, formerly Sendinblue

Zoho's transactional email API, priced per email with no monthly commitment