SMTP defines how a sending server establishes a connection, negotiates capabilities, and transmits a message envelope and body to a receiving server, with numeric response codes indicating success, temporary failure, or permanent rejection at each step.
Modern SMTP almost always runs with STARTTLS to encrypt the connection opportunistically, though opportunistic TLS can be silently downgraded by an interfering party unless the receiving domain enforces MTA-STS.
PulsePigeon's REST API sits in front of SMTP delivery for most integrations, so most customers never construct raw SMTP transactions directly, but the underlying delivery path is still standard SMTP end to end.