Definition
HELO and EHLO are the initial SMTP commands that a sending server uses to identify itself when connecting to a receiving mail server. HELO (Hello) is the basic command defined in the original SMTP specification in 1982. EHLO (Extended Hello) was introduced in the ESMTP specification in 1995 and signals that the sending server supports extended SMTP features such as TLS encryption, SMTP authentication, message size limits, and pipelining. Regardless of which command is used, the sending server transmits a hostname — typically the fully qualified domain name of the sending machine — and the receiving server can choose to accept or reject the connection based on this hostname.
The HELO or EHLO hostname is one of several signals that mailbox providers evaluate during the connection. For optimal deliverability, the hostname sent in the HELO/EHLO command must match the sending server's reverse DNS (PTR) record — the record that maps an IP address back to a hostname. If the PTR record does not match the HELO hostname, some receiving servers treat the connection as suspicious and may reject the message or apply additional spam scoring. Default HELO hostnames such as "localhost", "unknown", or generic hostnames from shared hosting platforms are common sources of deliverability problems. Proper HELO/EHLO configuration requires that the hostname sent during the SMTP handshake is a valid, resolvable domain name that matches both the PTR record and the domain used in the bounce address.
Best Practices
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Ensure your HELO/EHLO hostname matches your sending IP's PTR record exactly: The hostname your mail server sends during the SMTP handshake must match the hostname returned by a reverse DNS lookup of your sending IP. This is one of the most basic deliverability requirements. Use
dig -x <your-ip>to check your PTR record and compare it to your SMTP server's HELO configuration. -
Avoid generic HELO hostnames such as "localhost" or the server's internal hostname: Many shared hosting platforms and default server configurations send "localhost" or an internal hostname that does not resolve in public DNS. Mailbox providers treat these messages as potentially fraudulent. Configure your mail server to send a fully qualified domain name that resolves correctly.
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Use EHLO instead of HELO to signal extended SMTP capability: EHLO indicates that your server supports modern SMTP features such as STARTTLS for encryption. Receiving servers that see only HELO may assume your server is outdated and refuse connections or downgrade the delivery path. Modern email infrastructure should always use EHLO.
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Verify that your HELO hostname is not on any DNS blocklists: Even with proper configuration, if the domain used in your HELO hostname has a history of spam or was previously used for abusive purposes, it may appear on DNS-based blocklists. Check the HELO domain against major blocklists using tools such as MXToolbox before deploying a new sending IP.
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If using a shared sending platform, check how its HELO configuration works: Email service providers that offer shared IP pools typically manage HELO hostnames for you, but you should verify that the HELO hostname includes a domain you control or that the platform uses acceptable defaults. Some platforms allow custom HELO hostnames for dedicated IP addresses.
Related Glossary Terms
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Email Deliverability
Email deliverability is the ability to land in the recipient's inbox rather than the spam folder. It depends on sender reputation, authentication, list quality, and content practices.
Double Opt-In
Double opt-in (also called confirmed opt-in) is an email signup process that requires a new subscriber to confirm their email address by clicking a verification link in a confirmation email before they are added to your mailing list.
Email Alias
Alternative email addresses or subaddressing patterns that forward to a primary mailbox, affecting subscriber identity tracking and deduplication in email marketing.
Email Authentication
Email authentication (SPF, DKIM, DMARC) is a set of technical protocols that verify an email is genuinely from the domain it claims to be sent from. It prevents spoofing, phishing, and improves deliverability.
Frequently Asked Questions
HELO is the basic SMTP greeting command defined in the original SMTP standard. EHLO is the extended version introduced with ESMTP that allows the sending server to announce its capabilities such as TLS support, pipelining, and message size limits. Modern email servers should always use EHLO.
Mailbox providers check that the HELO hostname is a valid, resolvable domain that matches the sending IP's PTR record. A mismatch, a non-resolvable hostname, or a generic hostname such as "localhost" is associated with spam sending patterns and can cause messages to be rejected or filtered.
Use `dig -x
A PTR record, or reverse DNS record, maps an IP address to a hostname. When a receiving mail server does a reverse lookup on your sending IP, the returned hostname should match the hostname your server sent in the HELO/EHLO command. This consistency is a trust signal that your server is properly configured.
The HELO hostname should be a domain that you control and that is properly configured in DNS with forward and reverse resolution matching. Using a third-party domain without authorisation may cause deliverability issues and could be considered impersonation. Use a subdomain of your primary domain for best results.