transactional email
5 vendors in this category
Amazon SES
https://aws.amazon.com/ses/
Azure Communication Service
https://azure.microsoft.com/en-us/products/communication-services
CloudFlare Email Routing
https://www.cloudflare.com/developer-platform/products/email-routing/
Mandrill
https://mailchimp.com/features/transactional-email-infrastructure/
Postmark
postmarkapp.com