Several commands (RSET, DATA, QUIT) are specified as not permitting parameters.

The model for this is that distinct buffers are provided to hold the types of data objects; that is, there is a reverse-path buffer, a forward-path buffer, and a mail data buffer.

2.3.1. Mail Objects 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 (described in Section 3). It consists of an originator address (to Klensin Standards Track [Page 11] RFC 5321 SMTP October 2008 which error reports should be directed), one or more recipient addresses, and optional protocol extension material

The third step in the procedure is the DATA command (or some alternative specified in a service extension). DATA <CRLF>

When the RFC 822 format ([28], [4]) is being used, the mail data include the header fields such as those named Date, Subject, To, Cc, and From.

An SMTP-capable host SHOULD support both the alias and the list models of address expansion for multiple delivery. When a message is delivered or forwarded to each address of an expanded list form, the return address in the envelope ("MAIL FROM:") MUST be changed to be the address of a person or other entity who administers the list. However, in this case, the message header section (RFC 5322 [4]) MUST be left unchanged; in particular, the "From" field of the header section is unaffected.

To expand an alias, the recipient mailer simply replaces the pseudo- mailbox address in the envelope with each of the expanded addresses in turn; the rest of the envelope and the message body are left unchanged. The message is then delivered or forwarded to each expanded address.

The return (backward-pointing) address in the envelope is changed so that all error messages generated by the final deliveries will be returned to a list administrator, not to the message originator, who generally has no control over the contents of the list and will typically find error messages annoying.

If the Klensin Standards Track [Page 19] RFC 5321 SMTP October 2008 recipient is known not to be a deliverable address, the SMTP server returns a 550 reply, typically with a string such as "no such user - " and the mailbox name (other circumstances and reply codes are possible).

There exist mailing lists that perform additional, sometimes extensive, modifications to a message and its envelope. Such mailing lists need to be viewed as full MUAs, which accept a delivery and post a new message.

the reverse-path (originator) address specification command (MAIL)

The second step in the procedure is the RCPT command. This step of the procedure can be repeated any number of times.

An important mail facility is a mechanism for multi-destination delivery of a single message, by transforming (or "expanding" or "exploding") a pseudo-mailbox address into a list of destination mailbox addresses.

If the verb is initially accepted and the 354 reply issued, the DATA command should fail only if the mail transaction was incomplete (for example, no recipients),

5yz

Since it has been a common source of errors, it is worth noting that spaces are not permitted on either side of the colon following FROM in the MAIL command or TO in the RCPT command. The syntax is exactly as given above.

Similarly, servers MAY decline to accept mail that is destined for other hosts or systems.

An SMTP session is initiated when a client opens a connection to a server and the server responds with an opening message.

However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged.

Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command and extension name keywords) are not case sensitive, with the sole exception in this specification of a mailbox local-part (SMTP Extensions may explicitly specify case-sensitive elements). That is, a command verb, an argument value other than a mailbox local-part, and free form text MAY be encoded in upper case, lower case, or any mixture of upper and lower case with no impact on its meaning. The local-part of a mailbox MUST BE treated as case sensitive.

The terms "message content" and "mail data" are used interchangeably in this document to describe the material transmitted after the DATA command is accepted and before the end of data indication is transmitted.

RFC 3463 [25], specifies further structuring of the reply strings, including the use of supplemental and more specific completion codes (see also RFC 5248 [26]).

SMTP sessions are stateful

The means by which an SMTP client, once it has determined a target domain, determines the identity of an SMTP server to which a copy of a message is to be transferred, and then performs that transfer, is covered by this document. To effect a mail transfer to an SMTP server, an SMTP client establishes a two-way transmission channel to that SMTP server. An SMTP client determines the address of an appropriate host running an SMTP server by resolving a destination domain name to either an intermediate Mail eXchanger host or a final target host.

However, RFC 821 specifies some features that were not in significant use in the Internet by the mid-1990s and (in appendices) some additional transport models. Those sections are omitted here in the interest of clarity and brevity; readers needing them should refer to RFC 821.