Extensions
ESMTP is a protocol used to transport internet mail. It is used as both an inter-server transport protocol and (with restricted behavior enforced) a mail submission protocol.
The main identification feature is for ESMTP clients to open a transmission with the command EHLO (Extended HELLO), rather than HELO (Hello, the original RFC 821 standard). A server will respond with success (code 250), failure (code 550) or error (code 500, 501, 502, 504, or 421), depending on its configuration. An ESMTP server would return the code 250 OK in a multi-line reply with its domain and a list of keywords to indicate supported extensions. A RFC 821 compliant server would return error code 500, allowing ESMTP clients to try either HELO or QUIT.
Each service extension is defined in an approved format in subsequent RFCs and registered with the Internet Assigned Numbers Authority (IANA). The first definitions were the RFC 821 optional services - SEND, SOML (Send or Mail), SAML (Send and Mail), EXPN, HELP, and TURN. The format of additional SMTP verbs was set and for new parameters in MAIL and RCPT.
Some relatively common keywords (not all of them corresponding to commands) used today are:
8BITMIME— 8 bit data transmission, RFC 6152ATRN— AuthenticatedTURNfor On-Demand Mail Relay, RFC 2645AUTH— Authenticated SMTP, RFC 4954CHUNKING— Chunking, RFC 3030DSN— Delivery status notification, RFC 3461 (See Variable envelope return path)ETRN— Extended version of remote message queue starting commandTURN, RFC 1985HELP— Supply helpful information, RFC 821PIPELINING— Command pipelining, RFC 2920SIZE— Message size declaration, RFC 1870STARTTLS— Transport layer security, RFC 3207 (2002)SMTPUTF8— Allow UTF-8 encoding in mailbox names and header fields, RFC 6531
The ESMTP format was restated in RFC 2821 (superseding RFC 821) and updated to the latest definition in RFC 5321 in 2008. Support for the EHLO command in servers became mandatory, and HELO designated a required fallback.
Non-standard, unregistered, service extensions can be used by bilateral agreement, these services are indicated by an EHLO message keyword starting with "X", and with any additional parameters or verbs similarly marked.
SMTP commands are case-insensitive. They are presented here in capitalized form for emphasis only. An SMTP server that requires a specific capitalization method is a violation of the standard.
Read more about this topic: Extended SMTP
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