Greeting & extensions
03The server greets the client. EHLO establishes the client identity and discovers optional capabilities.
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
DIAGRAM§ 3.1 ↗
RFC 5321 · § 3.1EXAMPLE
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04
Choose supported features.
A client cannot assume an extension exists merely because it knows its name.
Protocol participantThreat or exceptionProtocol controlSimplified drawings · precise section references
Read the updated RFC section ↗
Use extensions only after capability discovery.
- A normal session starts with a 220 greeting followed by EHLO.
- The EHLO response advertises supported extension keywords and parameters.
- HELO remains a compatibility fallback when EHLO is not recognized; it does not negotiate extensions.
RFC 5321 defines the extension framework. STARTTLS, AUTH, SIZE, and other individual extensions have their own specifications and requirements.
Read the RFC section ↗Diagrams simplify the RFC. Read the linked sections for complete requirements.
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