HELO

What is HELO?

HELO (short for "hello") is the first thing a sending mail server says when it connects to a receiving mail server. It's how the sender introduces itself.

Along with the command, the sending server gives its hostname, for example HELO mail.example.com. Most servers today use the newer version of the command, EHLO ("extended hello"), but the two are usually just called HELO.

Why does HELO matter for deliverability?

Receiving servers use the HELO hostname as one of their earliest checks on whether a sender can be trusted. A good HELO hostname:

• Is a full hostname, like mail.example.com, not an IP address or a placeholder like localhost.
• Exists in DNS and points back to the IP address that's sending the email.
• Matches the reverse DNS of that IP address. Reverse DNS is a record that turns an IP address back into a hostname.

When these don't line up, it looks like a misconfigured or compromised server, which is common for spammers. Emails can be rejected or sent to spam before the receiving server has even looked at the message.

Spamhaus's Sender Guidelines say every sending IP should have a valid HELO and matching reverse DNS.

Do I need to set up HELO myself?

Usually not. If you send through an email service provider like EcoSend, the provider runs the sending servers, so HELO and reverse DNS are their responsibility.

Your job is the authentication tied to your own domain: SPF, DKIM and DMARC.

If you're choosing a provider, it's fair to ask whether they're ready for Spamhaus's HELO and reverse DNS requirement.

Example

A mail server at IP address 203.0.113.10 connects to Gmail and says EHLO mail.example.com.

Gmail looks up mail.example.com and finds it points to 203.0.113.10. It then looks up the reverse DNS for 203.0.113.10 and gets back mail.example.com. Everything matches, so the connection passes this check.

If the server had introduced itself as EHLO localhost, or its IP had no reverse DNS, many receiving servers would treat the email with suspicion or refuse it.

Read more in What Spamhaus wants from your emails.