ESMTP
also: Enhanced SMTP, SMTP Extensions
ESMTP (Extended Simple Mail Transfer Protocol) is an enhanced version of SMTP that adds modern capabilities like authentication, encryption, and larger message support to the original Simple Mail Transfer Protocol.
ESMTP extends the basic SMTP protocol with additional commands and features introduced in RFC 1869. It maintains backward compatibility with SMTP while enabling clients to negotiate advanced features before sending mail.
Key features include AUTH for user authentication, STARTTLS for encrypting the connection, support for 8-bit message content, and the ability to specify sender and recipient validation. Mail servers advertise ESMTP capabilities via the EHLO command instead of the basic HELO command.
For example, when you connect to a mail server on port 25 or 587, it responds with 220 mail.example.com ESMTP, allowing your client to use EHLO to discover supported features like AUTH LOGIN or STARTTLS before authenticating and sending email.