HELO String

A HELO string is the identifier sent by a mail server during the initial handshake of the Simple Mail Transfer Protocol (SMTP) communication. It introduces the sending server to the receiving server when establishing an email transmission session.

What Is a HELO String?

The HELO command (or EHLO for Extended HELO) is part of the SMTP protocol, which governs email transmission. When a sending server connects to a receiving mail server, it uses the HELO command to provide its domain name or IP address as an introduction.

This identifier helps the receiving server recognize the sending host and is often compared to Domain Name System (DNS) records for verification.

How Does a HELO String Work?

The HELO process works as follows:

1. SMTP connection starts: The sending mail server establishes a connection with the recipient’s server over SMTP (usually on port 25 or 587).
2. Greeting: The sending server sends the HELO or EHLO command along with its domain name or IP address.
3. Verification: The receiving server checks the HELO string against DNS records and IP reverse lookup for consistency.
4. Session continues: If accepted, the SMTP transaction proceeds with MAIL FROM, RCPT TO, and DATA commands to transmit the email.

An incorrect or suspicious HELO string (e.g., a mismatched domain or using an IP address instead of a domain) can raise red flags and cause the email to fail authentication or be marked as spam.

Why Is a HELO String Important?

The HELO string is critical because:

• Supports server identification: Allows the receiving system to validate the sender’s identity.
• Prevents spam and spoofing: Internet service providers (ISPs) and spam filters check the HELO string for authenticity.
• Impacts deliverability: Improper HELO configuration can lead to message rejection or spam folder placement.
• Ensures compliance: Aligning HELO with DNS records is part of email infrastructure best practices.

Without a proper HELO configuration, even legitimate emails may fail verification and hurt IP reputation.

Common Use Cases

HELO strings are relevant for:

• Mail server configuration: Making sure the hostname in HELO matches DNS PTR (reverse DNS) records.
• Anti-spam filtering: Spam detection systems often block emails with invalid or generic HELO strings.
• Email deliverability audits: Diagnosing issues with bounced or rejected emails due to HELO mismatch.

Example scenario: A business configures its mail server to use mail.company.com as the HELO string, matching its DNS and reverse DNS records, to avoid spam filtering issues.

FAQs About HELO String

What is the difference between HELO and EHLO?

EHLO is the extended version of HELO and is required for servers using Extended SMTP features such as authentication and encryption.

What happens if the HELO string is invalid?

Many receiving servers will reject the message or mark it as suspicious.

Should the HELO string match the domain?

Yes. It should match the fully-qualified domain name (FQDN) in your DNS records for best deliverability.

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