Simple Mail Transfer Protocol - the standard method computers use to send emails across the internet. Like the postal system's rules for how mail gets delivered.
When you click 'send' on an email, your email client uses SMTP to deliver the message to the recipient's email server.
SMTP is an email-sending protocol, not a cloud service. These offerings provide managed email sending that can be used via SMTP (or APIs), handling deliverability, scaling, and authentication.