HELO
The original SMTP greeting command (RFC 821), now superseded by EHLO but still accepted as a fallback.
HELO is the original SMTP greeting from RFC 821 (1982). A client says `HELO mail.example.com` to introduce itself and the server replies with a single line. Modern SMTP uses EHLO instead, which lets the server advertise extensions like STARTTLS and AUTH, but every SMTP server still accepts HELO as a fallback for ancient clients. The hostname the client claims in HELO/EHLO is logged and used in some anti-spam heuristics: it should match the sending IP's reverse DNS for best deliverability.