What Is the HELO Domain?

What is the HELO Domain?

The HELO domain (or EHLO domain in the extended SMTP version) is the domain name a mail server presents during the initial greeting of an SMTP session. This identifier helps the receiving mail server recognize the sender’s identity and can be used for SPF checks, logging, and reputation scoring. A properly configured HELO domain improves authentication reliability and deliverability.

The HELO command is the first step in the SMTP conversation between mail servers. When a sending server connects to a recipient, it announces itself using a domain name that ideally corresponds to its hostname or sending IP. The receiving server can then use this domain for reverse DNS lookups and policy evaluation.

How the HELO Domain Works

Example SMTP handshake:

In this example, mail.sender.com is the HELO domain. Modern systems use EHLO instead of HELO to indicate support for extended SMTP features.

The HELO or EHLO domain should:

• Match the sending server’s reverse DNS (PTR) record
• Resolve to a valid A or AAAA record
• Represent a domain under the sender’s control

Consistency between the HELO domain, PTR record, and SPF configuration helps establish legitimacy and reduces the likelihood of spam filtering.

The Role of the HELO Domain in Email Authentication

Receiving mail systems often evaluate the HELO domain as part of their anti-spam and authentication checks. Invalid or generic HELO domains (such as “localhost” or an unrelated hostname) can trigger reputation penalties or message rejection.

Properly configured HELO domains support:

• Stronger SPF authentication alignment
• Accurate mail server identification in logs
• Better deliverability and fewer spam flagging events
• Alignment with reverse DNS and sending IP reputation

HELO Domain and DMARCeye

DMARCeye analyzes HELO and EHLO domains appearing in authentication reports to help identify which mail servers are sending on behalf of your domain. By correlating these identifiers with DMARC aggregate data and IP reputation, DMARCeye makes it easy to spot misconfigured or unauthorized servers that may harm deliverability or security.

Consistent HELO domain configuration across all sending sources supports clean authentication paths, strong domain reputation, and reliable DMARC enforcement.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.