What is HELO and EHLO?
Also called: HELO, EHLO, SMTP greeting
HELO and EHLO are the first commands in an SMTP conversation, in which the connecting server introduces itself by hostname. EHLO is the extended version: the receiving server replies with the features it supports, such as STARTTLS and size limits. The name given is checked as part of the sender's identity.
RFC 5321 says the client should send its fully qualified domain name. Receiving servers compare that name with the connecting IP's reverse DNS and with the forward DNS of the name itself. A server that introduces itself as localhost, as a bare IP, or as a hostname that does not resolve looks like a compromised machine.
For senders, the HELO name is usually set by your sending platform and is worth checking only on your own servers or a dedicated IP. For verification hosts it matters a great deal: a probe from a host with a mismatched identity is more likely to be greylisted, slowed or refused.
SPF can also be evaluated against the HELO identity, which is one more reason the name should be a real hostname you control.
Example
EHLO mail.example.com → 250-mx.google.com at your service / 250-SIZE 157286400 / 250-STARTTLSGo deeper
Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms