EHLOHELO

EHLOHELO is an informal term used in some networking discussions to describe the handling of the initial SMTP greeting when a client connects to a mail server. It refers to situations where an implementation supports both the extended greeting EHLO and the legacy HELO commands, or where greeting negotiation blends aspects of both approaches. The term is not part of an official standard but is used to discuss dual-mode or backward-compatible SMTP behavior.

Background and relationship to SMTP: In the Simple Mail Transfer Protocol, HELO is the original greeting command, while EHLO introduces extended capabilities that can be advertised to the client (such as STARTTLS, 8BITMIME, SMTPUTF8). A practical interpretation of EHLOHELO is a dual-mode greeting capability in which a client may attempt EHLO first to discover extensions, and gracefully fall back to HELO if the server does not support EHLO or its extensions. This approach aims to maximize compatibility with a mix of legacy and modern servers.

Operation: In a typical flow, a client sends EHLO domain. The server may respond with a 250 status code and a list of supported extensions. If EHLO is not supported, the client can retry with HELO. After a successful greeting, the SMTP transaction proceeds with mail commands such as MAIL FROM, RCPT TO, and DATA, with security considerations like STARTTLS applying when offered by the server.

Security and compatibility: The EHLOHELO concept highlights the importance of graceful fallback, TLS negotiation, and adherence to RFC 5321 and related standards to ensure interoperable communication across diverse servers.

See also: SMTP, EHLO, HELO, ESMTP.