RFC821-Simple Mail Transfer Protocol SMTP Chinese version-1. 3. 7. the domain was recently introduced to the ARPAInternet email system. The domain can be used to change the address space from a flat string host name to a global address level. Host 3.7. the domain was recently introduced to the ARPA Internet mail system. The domain can be used to change the address space from a flat string host name to a global address level. A host is represented by a domain name. The starting host is composed of a series of meta strings, which are arranged by commas in the most special to general order. For example, "USC-ISIF.ARPA", "Fred. Cambridge. UK", and "PC7.LCS. MIT. ARPA" can be host-domain identifiers. No matter how the domain name is used in SMTP, only the formal name can be used, and the alias or nickname cannot be used. 3.8. The change role TURN command can be used to change the role of the program for communication over the transmission channel. If program A is sending SMTP now, it sends the TURN command and receives the OK response (250), then it becomes receiving SMTP. Similarly, program B can also change from receiving SMTP to sending SMTP. If you want to refuse to change the role, the recipient can send a 502 message as a response. Note: this command is optional. This command is generally not used when a TCP transmission channel is used. However, this command is useful when the cost of establishing a transmission channel is relatively high. For example, this command supports a common public exchange telephone system as a transmission channel. 4. SMTP description 4.1. SMTP command 4.1.1. command syntax SMTP command defines mail transmission or user-defined system functions. Its command is composed End string. In the case of parameters Separate from parameters. Connection. The syntax format of the mailbox must be the same as that of the receiving site. Next we will discuss SMTP commands and responses. The email sending operation involves different data objects, which are connected by different parameters. The reply path is the MAIL command parameter, while the forward path is the RCPT command parameter, and the MAIL date is the DATA command parameter. These parameters or data objects must follow the command. In this mode, different buffers are required to store these objects. that is to say, there is a reply path buffer, a forward path buffer, and a mail content buffer. A specific command generates its own buffer, or clears one or more Buffered content. HELLO (HELO) This command is used to send SMTP confirmation to receive SMTP. The parameter field includes the SMTP sending host name. Receive SMTP send SMTP confirmation to receive SMTP through the connection confirmation command. Commands and OK responses confirm that sending and receiving SMTP enters the initial state, that is, no operation is being executed, and all status tables and buffers are cleared. MAIL command is used to start sending emails to multiple mailboxes. The parameter field includes the reply path. The returned path contains the list of optional host and sender email addresses. When there is a host list, it is a reply path source, it indicates that the mailbox is sent by the host in the table (the first host is the last host that receives the email. This table also provides the source path for returning non-pass message numbers to the sender. Because each transfer host address is added at the beginning of this table, it must use a clear name for sending IPCE instead of for receiving IPCE (if they are not an IPCE. Some error message reply paths may be empty. This command clears the reply path buffer, the forward path buffer, and the Mail content buffer, and inserts the reply path information of this command into the reply path buffer. RECIPIENT (RCPT) This command is used to determine the unique RECIPIENT of the Mail content; multiple recipients will be specified by multiple this command. The forwarding path contains an optional host and a required destination mailbox. When the host list appears, this is a source path, which indicates that the email must be sent to the previous host in the list. If the receiving SMTP fails to deliver the mail, the system will return information such as unknown local user (550) to the user. When an email is sent, the transmission host must move its name from the beginning of the forwarding path to the end of the reply path. When the Mail finally arrives at the destination, the received SMTP will be inserted into the target mail with its own name in its host Mail format. For example, when an email received by the transfer host A contains the following parameters, FROM: TO: <@ HOSTA. ARPA, @ HOSTB. ARPA: USERC@HOSTD.ARPA> will become the form below: FROM: <@ HOSTA. ARPA: USERX@HOSTY.ARPA> TO: <@ HOSTB. ARPA: USERC@HOSTD.ARPA>. this command causes its forwarding path parameters to be added to the forwarding path buffer. The DATA (DATA) recipient uses the line following the command as the Mail content. This command causes the Mail content after this command to be added to the Mail content buffer. The email content can contain all 128 ASCII characters. The Mail content ends with only one full stop line, that is, the following character sequence :" . ", It indicates the end of the email. The end instruction of the Mail content requires the recipient to process the saved mail content now. This process clears the reply path buffer, the forward path buffer, and the Mail content buffer. If the operation succeeds, the receiver must return an OK response. if the operation fails, the receiver must also return a failed response. When receiving a message from SMTP, whether it is used for forwarding or the email has arrived at the destination, it must add a timestamp line at the beginning of the email content, this line indicates the identity of the receiving master machine and the sending host, as well as the time and date of receiving the Mail content. The forwarded mail will have timestamps such as multiple lines. When receiving SMTP as the last transfer, it inserts the returned path information row into the mail. This line includes . Here, the last transfer means that the Mail will be sent to the target user, but in some cases, emails can be further processed and sent by another email system. The email address in the returned path may be different from the email address actually sent. This may occur when a specific error handling mailbox needs to be sent, rather than the sender. As described above, the final mail content consists of one return path line and one or more timestamp lines following it. These lines are followed by the header and body information of the Mail content. A specific description is required when the Mail data instruction section after processing is successful. This situation may occur when sending SMTP and finding that when the Mail needs to be sent to multiple users, it can only successfully send information to one part of it. In this case, you must send an OK response to the DATA command, and then receive the SMTP organization and send a "non-deliverable mail" message to the message sender. In this information, either a list of unsuccessful recipients is sent, or a list of unsuccessful recipients is sent multiple times instead of a receiver each time. All MAIL messages that cannot be passed are sent by the MAIL command. Return-Path: <@ GHI. ARPA, @ DEF. ARPA, @ ABC. ARPA: JOE@ABC.ARPA> pinned Ed: from GHI. ARPA by JKL. ARPA; 27 Oct 81 15:27:39 PST partitioned Ed: from DEF. ARPA by GHI. ARPA; 27 Oct 81 15:15:13 PST Received: from ABC. ARPA by DEF. ARPA; 27 Oct 81 15:01:59 PST Date: 27 Oct 81 15:01:01 PST From: JOE@ABC.ARPA Subject: Improved Mailing System Installed To: SAM@JKL.ARPA This is to inform you that... SEND (S END) This command is used to start a sending command to send emails to one or more terminals. The parameter field contains a reply path. if this command is successful, the Mail will be sent to the terminal. The reply path includes an optional host list and the sender's email address. When the host list appears, it indicates that this is a transfer path, and the mail is sent to each host on this path (the first host in the list is the last host ). This table is used to return non-transmitted signals to the sender. Because each transfer host address is added at the beginning of this table, it must use a clear name for sending IPCE instead of receiving IPCE (if they are not an IPCE. The reply path for some error messages may be empty. This command clears the reply path buffer, the forward path buffer, and the Mail content buffer, and inserts the reply path information of this command into the reply path buffer. Send or mail (SOML) This command is used to start a MAIL operation to SEND the MAIL content to one OR more terminals, OR to the mailbox. For each recipient, if the recipient terminal opens, the Mail content will be sent to the recipient's terminal, otherwise it will be sent to the recipient's mailbox. The parameter field includes the reply path. if the information is successfully sent to the terminal or mailbox, this command is successful. The reply path includes an optional host list and the sender's email address. When the host list appears, the table shows that this is a transfer path, and the mail is sent to each host on this path (the first host in the list is the last host ). This table is used to return non-transmitted signals to the sender. Because each transfer host address is added at the beginning of this table, it must use a clear name for sending IPCE instead of for receiving IPCE (if they are not an IPCE. The reply path of some error messages can be empty. This command clears the reply path buffer, the forward path buffer, and the Mail content buffer, and inserts the reply path information of this command into the reply path buffer. Send and mail (SAML) This command is used to start a MAIL operation to SEND the MAIL content to one or more terminals and send it to the mailbox. If the recipient terminal opens, the Mail content will be sent to the recipient's terminal and the recipient's mailbox. The parameter field includes the reply path. if the information is successfully sent to the mailbox, this command is used. The reply path includes an optional host list and the sender's email address. When the host list appears, the table shows that this is a transfer path, and the mail is sent to each host on this path (the first host in the list is the last host ). This table is used to return non-transmitted signals to the sender. Because each transfer host address is added at the beginning of this table, it must use a clear name for sending IPCE instead of for receiving IPCE (if they are not an IPCE. The reply path of some error messages can be empty. This command clears the reply path buffer, the forward path buffer, and the Mail content buffer, and inserts the reply path information of this command into the reply path buffer. RESET (RSET) this command indicates that the Mail sending operation will be abandoned. Any stored senders, recipients, and email content should be discarded, and all buffer and status tables should be cleared. the recipient must return an OK response. VERIFY (VRFY) this command requires the recipient to confirm that the parameter is a user. If this is (known) the user name, return the user's full name and the specified mailbox. This command has no effect on the buffer of the reply path, the buffer of the forwarding path, and the buffer of the Mail content. EXPAND (EXPN) this command requires the recipient to confirm that the parameter specifies a mail sending list. if it is a mail sending list, it returns the members in the table. If this is (known) the user name, return the user's full name and the specified mailbox. This command does not affect the reply path buffer, the forwarding path buffer, and the Mail content buffer. HELP (HELP) this command causes the recipient to send HELP information to the HELP command sender. This command can contain parameters and return specific information as a response. This command does not affect the reply path buffer, the forwarding path buffer, and the Mail content buffer. NOOP (NOOP) this command does not affect any parameters and commands that have been issued. It only indicates that there is no operation, not that the recipient has sent an OK response. This command is used for the reply path buffer, forward path buffer, and Mail
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