HELO
The original SMTP greeting command from RFC 821: it identifies the client to the server but cannot negotiate any SMTP extensions.
What it means
HELO does the job of EHLO without the extension list: the server answers with a single 250 line and the session continues in plain SMTP. RFC 5321 still requires every server to accept it, so older clients keep working.
A client that says HELO gives up STARTTLS, SMTP AUTH, SIZE, 8BITMIME and every other extension, which is why modern sending software uses EHLO. The name given in either command is what SPF calls the HELO identity, and what receivers compare with the connecting IP and its reverse DNS, so the same hostname rules apply whichever command is used.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 4.1.1.1: Extended HELLO (EHLO) or HELLO (HELO)rfc-editor.org
- RFC 7208, section 2.3: The "HELO" Identityrfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes