HELO vs EHLO: Why Your SMTP Greeting Can Hurt Email Delivery

Your SPF, DKIM, and DMARC records may all look correct while your mail server still introduces itself as:

EHLO localhost

or:

EHLO server1.internal

That greeting happens before the message body is transmitted and can become one of the first identity signals a receiving server evaluates.

What HELO and EHLO actually do

When one mail server connects to another, the receiving server first sends an SMTP banner:

220 mx.receiver.example ESMTP

The sending server then identifies itself:

EHLO mail.sender.example

HELO is the older SMTP greeting. EHLO is the modern Extended SMTP greeting. It also allows the receiving server to advertise capabilities such as:

• STARTTLS
• SIZE
• 8BITMIME
• PIPELINING
• DSN

Modern mail servers normally try EHLO first and fall back to HELO only when necessary.

What a valid EHLO hostname should look like

A good EHLO identity