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EHLO and HELO

HELO and EHLO are commands used to initiate a session between a Mail User Agent (MUA) or Mail Transfer Agent (MTA) and an SMTP server.^[600-developer-smtp.md]

HELO

The HELO command is the first instruction sent to a standard SMTP server.^[600-developer-smtp.md] It is used to "greet" the server and identify the client via the domain name provided in the command.^[600-developer-smtp.md] This domain name is frequently utilized in anti-spam checks; therefore, it must be a resolvable Fully Qualified Domain Name (FQDN) that matches the client's current IP address.^[600-developer-smtp.md]

EHLO

The EHLO command is the equivalent greeting used for extended SMTP (ESMTP) servers.^[600-developer-smtp.md] Like HELO, it identifies the client's domain name, which is subject to the same anti-spam validation requirements.^[600-developer-smtp.md] The key difference is that in response to an EHLO command, the server will reply with a list of supported ESMTP extensions, such as STARTTLS.^[600-developer-smtp.md]

Sources

^[600-developer-smtp.md]