|
Extended SMTP (ESMTP), sometimes referred to as Enhanced SMTP, is a definition of protocol extensions to the Simple Mail Transfer Protocol standard. The extension format was defined in November 1995 in IETF publication RFC 1869 which established a general structure for all existing and future extensions. ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions. ==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 6152* ATRN — Authenticated TURN for On-Demand Mail Relay, RFC 2645 * AUTH — Authenticated SMTP, RFC 4954* CHUNKING — Chunking, RFC 3030* DSN — Delivery status notification, RFC 3461 (See Variable envelope return path)* ETRN — Extended version of remote message queue starting command TURN, RFC 1985* HELP — Supply helpful information, RFC 821* PIPELINING — Command pipelining, RFC 2920 * SIZE — Message size declaration, RFC 1870* STARTTLS — Transport layer security, RFC 3207 (2002)* SMTPUTF8 — Allow UTF-8 encoding in mailbox names and header fields, RFC 6531* UTF8SMTP — Allow UTF-8 encoding in mailbox names and header fields, RFC 5336 (deprecated〔(【引用サイトリンク】url=http://www.iana.org/assignments/mail-parameters/mail-parameters.txt )〕)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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Extended SMTP」の詳細全文を読む スポンサード リンク
|