SMTP HELO/EHLO
HELO is the command an SMTP client sends to identify itself at the start of a mail session, defined in the original SMTP standard; EHLO is the extended version, introduced by RFC 1869 and carried forward in RFC 5321, that a client sends instead to ask the server which SMTP extensions it supports. A server that receives EHLO replies with a multi-line list of capabilities -- STARTTLS, AUTH, SIZE, PIPELINING, 8BITMIME, and others -- that the client can then actually use during that session, whereas a server that only understands plain HELO limits the session to the base SMTP command set with none of those extensions available. Essentially every modern mail server speaks EHLO; a server or client falling back to bare HELO today usually signals either deliberately minimal tooling or genuinely old software, and a diagnostic SMTP connection is typically opened with EHLO specifically to see that capability list before deciding how to proceed.
By AdminKit team, Netbase JSC · Updated