HELO

HELO is a command used in the Simple Mail Transfer Protocol (SMTP) for identifying the sending client to a receiving mail server at the start of a session. Historically, HELO is short for "Hello" and was defined in early SMTP specifications. In modern practice, Extended SMTP (ESMTP) uses the EHLO command to identify the client and to advertise the server’s capabilities, such as authentication and encryption options. HELO remains available primarily for backward compatibility, but many servers treat it with tighter checks or may require EHLO for full functionality.

In a typical SMTP exchange, a client connects to a mail server on port 25 (or submission port 587) and issues either HELO <domain> or EHLO <domain> to identify itself. The server responds with a status code, commonly 250 if the identification is accepted. When EHLO is used, the server’s response usually lists relevant extensions supported by the server, such as STARTTLS, AUTH, SIZE, and 8BITMIME. If the identification string is malformed or unresolved, the server may respond with error codes such as 500, 501, or other negative replies.

Security and policy considerations often influence the use of HELO/EHLO. The identity provided by the client’s domain should resolve in DNS and ideally match the sending host’s IP address (reverse DNS). Mismatches or suspicious identifiers can trigger spam filters or rejection. Modern SMTP implementations typically prefer EHLO due to its support for extended features and improved interoperability.