https://serversmtp.com/ https://serversmtp.com/ It is the only one I have used, it offers both SMTP and a REST API of which I have only used REST, at $COMPANY

https://serversmtp.com/ https://serversmtp.com/ It is the only one I have used, it offers both SMTP and a REST API of which I have only used REST, at $COMPANY it was valued as "the cheapest working service to send thousands of email/month from a few email address (the usual noreply@xxx.yy)"