HELO hostname
The hostname a sending server gives in its EHLO or HELO command, which receivers record in the Received header and often check against DNS and the connecting IP.
What it means
RFC 5321 asks the client to give its fully qualified domain name, and an address literal in square brackets only when it has no meaningful name. Receivers check that name in several ways: whether it resolves, whether it matches the reverse DNS (PTR) of the connecting IP, and, in SPF, as the HELO identity. Gmail rejects an invalid or empty HELO name with 501 5.5.4, and Microsoft lists a bad HELO name among its rejection reasons.
The tidy setup is one name per sending IP: a hostname whose A or AAAA record points to the IP, the IP's PTR record pointing back to the same hostname, and that hostname used in EHLO. Names such as localhost, or an IP address written without the brackets of an address literal, invite rejection.
Example
EHLO mta1.mail.example.comRelated terms
On this site
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
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
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