“The objective of the Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently.”

“When an SMTP client has a message to transmit, it establishes a two-way transmission channel to an SMTP server. The responsibility of an SMTP client is to transfer mail messages to one or more SMTP servers, or report its failure to do so.”

“An SMTP server may be either the ultimate destination or an intermediate "relay" (that is, it may assume the role of an SMTP client after receiving the message) or "gateway" (that is, it may transport the message further using some protocol other than SMTP).”

“SMTP commands are generated by the SMTP client and sent to the SMTP server. SMTP replies are sent from the SMTP server to the SMTP client in response to the commands.”

“Once the transmission channel is established and initial handshaking is completed, the SMTP client normally initiates a mail transaction.”

“Such a transaction consists of a series of commands to specify the originator and destination of the mail and transmission of the message content (including any lines in the header section or other structure) itself.”

“The server responds to each command with a reply; replies may indicate that the command was accepted, that additional commands are expected, or that a temporary or permanent error condition exists.”

“As suggested above, this protocol provides mechanisms for the transmission of mail. Historically, this transmission normally occurred directly from the sending user's host to the receiving user's host when the two hosts are connected to the same transport service.”

“When they are not connected to the same transport service, transmission occurs via one or more relay SMTP servers.”

“A very common case in the Internet today involves submission of the original message to an intermediate, "message submission" server, which is similar to a relay but has some additional properties”

“An intermediate host that acts as either an SMTP relay or as a gateway into some other transmission environment is usually selected through the use of the domain name service (DNS) Mail eXchanger mechanism.”

“In an effort that started in 1990, approximately a decade after RFC 821 was completed, the protocol was modified with a "service extensions" model that permits the client and server to agree to utilize shared functionality beyond the original SMTP requirements.”

“SMTP's strength comes primarily from its simplicity. Experience with many protocols has shown that protocols with few options tend towards ubiquity, whereas protocols with many options tend towards obscurity.”

“In many cases, the cost of extending the SMTP service will likely outweigh the benefit.”

“The IANA maintains a registry of SMTP service extensions. A corresponding EHLO keyword value is associated with each extension.”

“In particular, extensions can change the minimum limits specified in Section 4.5.3, can change the ASCII character set requirement as mentioned above, or can introduce some optional modes of message handling.”

“In particular, if an extension implies that the delivery path normally supports special features of that extension, and an intermediate SMTP system finds a next hop that does not support the required extension…

it MAY choose, based on the specific extension and circumstances, to requeue the message and try later and/or try an alternate MX host.”

“SMTP transports a mail object. A mail object contains an envelope and content.”

“The SMTP envelope is sent as a series of SMTP protocol units”

“It consists of an originator address (to which error reports should be directed), one or more recipient addresses, and optional protocol extension material.”

“The SMTP content is sent in the SMTP DATA protocol unit and has two parts: the header section and the body.”

“If the content conforms to other contemporary standards, the header section consists of a collection of header fields, each consisting of a header name, a colon, and data, structured as in the message format specification; the body, if structured, is defined according to MIME”

“The content is textual in nature, expressed using the US-ASCII repertoire. Although SMTP extensions may relax this restriction for the content body, the content header fields are always encoded using the US-ASCII repertoire.”