EHLO
The extended SMTP greeting a client sends to advertise support for SMTP extensions like STARTTLS, SIZE, and AUTH.
EHLO (Extended HELLO) is the first command an SMTP client sends after connecting. It replaces the original HELO from RFC 821 and tells the server `I speak Extended SMTP`. The server responds with a multi-line list of supported extensions: STARTTLS, 8BITMIME, SIZE, AUTH, PIPELINING, and more. Modern mail flow depends on EHLO succeeding, since STARTTLS (and therefore transport encryption) is only negotiated after the extension is advertised. A server that rejects EHLO and only accepts HELO is effectively legacy-only.