Information about Smtp Extension
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 RFC 1869 in 1995.
RFC 1869 established a structure for all existing and future extensions, to produce a consistent and manageable means by which ESMTP clients and servers can be identified and ESMTP servers can indicate supported extensions to connected clients.
Each service extension is defined in an approved format in subsequent RFCs and registered with the 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:
Non-standard, unregistered, service extensions can be used by bilateral agreement, these services are indicated by an EHLO keyword starting with "X", and with any additional parameters or verbs similarly marked.
Although commands are listed in upper case in this article, it is not necessary and an SMTP server that requires upper case is a violation of RFCs 821 and 2821. The MUST is for commands to be in US-ASCII and terminated by <crlf> or <sp> and parameters.
RFC 1869 established a structure for all existing and future extensions, to produce a consistent and manageable means by which ESMTP clients and servers can be identified and ESMTP servers can indicate supported extensions to connected clients.
Extensions
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 can then 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. An RFC 821 compliant server would return error code 500, allowing the ESMTP client to try either HELO or QUIT.Each service extension is defined in an approved format in subsequent RFCs and registered with the 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 1652
- ATRN — Authenticated Turn, RFC 2645
- SMTP-AUTH — Authenticated SMTP, RFC 2554
- CHUNKING — Chunking, RFC 3030
- DSN — Delivery status notification, RFC 1891
- ETRN — Extended 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
Non-standard, unregistered, service extensions can be used by bilateral agreement, these services are indicated by an EHLO keyword starting with "X", and with any additional parameters or verbs similarly marked.
Although commands are listed in upper case in this article, it is not necessary and an SMTP server that requires upper case is a violation of RFCs 821 and 2821. The MUST is for commands to be in US-ASCII and terminated by <crlf> or <sp> and parameters.
External links
- IANA registry of mail parameters includes service extension keywords
Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3) chapter 5. The protocol used today is also known as ESMTP and defined in RFC 2821.
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Internet Assigned Numbers Authority (IANA) is the entity that oversees global IP address allocation, DNS root zone management, and other Internet protocol assignments. It is operated by ICANN.
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8BITMIME (RFC 1652) is an SMTP extension standardized in 1994 that facilitates the exchange of e-mail messages containing octets outside the seven-bit ASCII range. Prior to the availability of 8BITMIME implementations, mail user agents employed several techniques to cope with the
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SMTP-AUTH is an extension of the Simple Mail Transfer Protocol (SMTP) to include an authentication step through which the client effectively logs in to the mail server during the process of sending mail.
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Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers.
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American Standard Code for Information Interchange (ASCII), generally pronounced ask-ee IPA: /ˈæski/ ( [1] ), is a character encoding based on the English alphabet.
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