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Simple Mail Transfer Protocol and pop3 protocal
1... INTRODUCTION
The objective of Simple Mail Transfer Protocol (SMTP) is to transfer mail reliably and efficiently. SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel.
An important feature of SMTP is its capability to relay mail across transport service environments. A transport service provides an interposes communication environment (IPCE). An IPCE may cover one network, several networks, or a subset of a network. It is important to realize that transport systems (or IPCEs) are not one-to-one with networks. A process can communicate directly with another process through any mutually known IPCE. Mail is an application or use of interposes communication. Mail can be communicated between processes in different IPCEs by relaying through a process connected to two (or more) IPCEs. More specifically, mail can be relayed between hosts on different transport systems by a host on both transport systems.
2. THE SMTP MODEL
The SMTP design is based on the following model of communication: as the result of a user mail request, the sender-SMTP establishes a two-way transmission channel to a receiver-SMTP. The receiver-SMTP may be either the ultimate destination or an intermediate. SMTP commands are generated by the sender-SMTP and sent to the receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the sender-SMTP in response to the commands.
Once the transmission channel is established, the SMTP-sender sends a MAIL command indicating the sender of the mail. If the SMTP-receiver can accept mail it responds with an OK reply. The SMTP-sender then sends a RCPT command identifying a recipient of the mail. If the
SMTP-receiver can accept mail for that recipient it responds with an OK reply; if not, it responds with a reply rejecting that recipient (but not the
Whole mail transaction). The SMTP-sender and SMTP-receiver may negotiate several recipients. When the recipients have been negotiated the SMTP-sender sends the mail data, terminating with a special sequence. If the SMTP-receiver successfully processes the mail data it responds with an OK reply. The dialog is purposely lock-step, one-at-a-time.
+----------+ +----------+ +------+ | | | | | User |<-->| | SMTP | | +------+ | Sender- |Commands/Replies| Receiver-| +------+ | SMTP |<-------------->| SMTP | +------+ | File |<-->| | and Mail | |<-->| File | |System| | | | | |System| +------+ +----------+ +----------+ +------+ Sender-SMTP Receiver-SMTP Model for SMTP Use Figure 1 -------------------------------------------------------
The SMTP provides mechanisms for the transmission of mail; directly from the sending user's host to the receiving user's host when the August 1982 Simple Mail Transfer Protocol two host are connected to the same transport service, or via one or more relay SMTP-servers when the source and destination hosts are not connected to the same transport service.
To be able to provide the relay capability the SMTP-server must be supplied with the name of the ultimate destination host as well as the destination mailbox name.
The argument to the MAIL command is a reverse-path, which specifies who the mail is from. The argument to the RCPT command is a forward-path, which specifies who the mail is to. The forward-path is a source route, while the reverse-path is a return route (which may be
Used to return a message to the sender when an error occurs with a relayed message).
When the same message is sent to multiple recipients the SMTP encourages the transmission of only one copy of the data for all the recipients at the same destination host. The mail commands and replies have a rigid syntax. Replies also have a numeric code. Commands and replies are not case sensitive. That is, a command or reply word may be upper case, lower case, or any mixture of upper and lower case. Note that this is not true of mailbox user names. For some hosts the user name is case sensitive, and SMTP implementations must take case to preserve the case of user names as they appear in mailbox arguments. Host names are not case sensitive.
Commands and replies are composed of characters from the ASCII character set [1]. When the transport service provides an 8-bit byte (octet) transmission channel, each 7-bit character is transmitted right justified in an octet with the high order bit cleared to zero.
When specifying the general form of a command or reply, an argument or special symbol will be denoted by a meta-linguistic variable (or constant), for example,"" or "". Here the angle brackets indicate these are meta-linguistic variables.
However, some arguments use the angle brackets terally. For example, an actual reverse-path is enclosed in angle brackets, i.e.,"" is an instance of (the angle brackets are actually transmitted in the command or reply).
3 THE SMTP PROCEDURES
This section presents the procedures used in SMTP in several parts. First comes the basic mail procedure defined as a mail transaction. Following this are descriptions of forwarding mail, verifying mailbox names and expanding mailing lists, sending to terminals instead of or in combination with mailboxes, and the opening and closing exchanges. At the end of this section are comments on relaying, a note on mail domains, and a discussion of changing roles.
3.1 MAIL
There are three steps to SMTP mail transactions. The transaction is started with a MAIL command which gives the sender identification. A series of one or more RCPT commands follows giving the receiver information. Then a DATA command gives the mail data. And finally, the end of mail data indicator confirms the transaction.
The first step in the procedure is the MAIL command. The contains the source mailbox.
MAIL FROM :< reverse-path>
This command tells the SMTP-receiver that a new mail transaction is starting and to reset all its state tables and buffers, including any recipients or mail data. It gives the reverse-path which can be used to report errors. If accepted, the receiver-SMTP returns a 250 OK reply.
The can contain more than just a mailbox. The is a reverse source outing list of hosts and source mailbox. The first host in the should be the host sending this command.
The second step in the procedure is the RCPT command. RCPT TO :< forward-path>
This command gives a forward-path identifying one recipient. If accepted, the receiver-SMTP returns a 250 OK reply, and stores the forward-path. If the recipient is unknown the receiver-SMTP returns a 550 Failure reply. This second step of the procedure can be repeated any number of times.
The can contain more than just a mailbox. The is a source routing list of hosts and the destination mailbox. The first host in the should be the host receiving this command.
The third step in the procedure is the DATA command.
DATA
If accepted, the receiver-SMTP returns a 354 Intermediate reply and considers all succeeding lines to be the message text. When the end of text is received and stored the SMTP-receiver sends a 250 OK reply. Since the mail data is sent on the transmission channel the end of the mail data must be indicated so that the command and reply dialog can be resumed. SMTP indicates the end of the mail data by sending a line containing only a period. A transparency procedure is used to prevent this from interfering with the user's text.
Please note that the mail data includes the memo header items such as Date, Subject, To, Cc, from [2].The end of mail data indicator also confirms the mail transaction and tells the receiver-
SMTP to now process the stored recipients and mail data. If accepted, the receiver-SMTP returns a 250 OK reply. The DATA command should fail only if the mail transaction was incomplete (for example, no recipients), or if resources are not available.
The above procedure is an example of a mail transaction. These commands must be used only in the order discussed above. Example 1 (below) illustrates the use of these commands in a mail transaction.
Example of the SMTP Procedure
This SMTP example shows mail sent by Smith at host Alpha.ARPA, to Jones, Green, and Brown at host Beta.ARPA. Here we assume that host Alpha contacts host Beta directly.
S: MAIL FROM :< Smith@Alpha.ARPA> R: 250 OK S: RCPT TO :< Jones@Beta.ARPA> R: 250 OK S: RCPT TO :< Green@Beta.ARPA> R: 550 No such user here S: RCPT TO :< Brown@Beta.ARPA> R: 250 OK S: DATA R: 354 Start mail input; end with .
S: Blah blah blah...
S: ...etc. etc. etc. S: . R: 250 OK
The mail has now been accepted for Jones and Brown. Green did not have a mailbox at host Beta in Example 1.
3.2. FORWARDING
There are some cases where the destination information in the is incorrect, but the receiver-SMTP knows the correct destination. In such cases, one of the following replies should be used to allow the sender to contact the correct destination.
251 -User not local; will forward to
This reply indicates that the receiver-SMTP knows the user's mailbox is on another host and indicates the correct forward-path to use in the future. Note that either the host or user or both may be different. The receiver takes responsibility for delivering the message.
551 User not local; please try
This reply indicates that the receiver-SMTP knows the user's mailbox is on another host and indicates the correct forward-path to use. Note that either the host or user or both may be different. The receiver refuses to accept mail for this user, and the sender must either redirect the mail according to the information provided or return an error response to the originating user.
3.3 VERIFYING AND EXPANDING
SMTP provides as additional features, commands to verify a user name or expand a mailing list. This is done with the VRFY and EXPN commands, which have character string arguments. For the VRFY command, the string is a user name, and the response may include the full name of the user and must include the mailbox of the user. For the EXPN command, the string identifies a mailing list, and the multilane response may include the full name of the users and must give the mailboxes on the mailing list. "User name" is a fuzzy term and used purposely. If a host implements the VRFY or EXPN commands then at least local mailboxes must be recognized as "user names". If a host chooses to recognize other strings as "user names" that is allowed.
In some hosts the distinction between a mailing list and an alias for a single mailbox is a bit fuzzy, since a common data structure may hold both types of entries, and it is possible to have mailing lists of one mailbox. If a request is made to verify a mailing list a positive response can be given if on receipt of a message so addressed it will be delivered to everyone on the list, otherwise an error should be reported (e.g., "550 That is a mailing list, not a user"). If a request is made to expand a user name returning a list containing one name can form a positive response, or an error can be reported (e.g., "550 That is a user name, not a mailing list").
In the case of a multiline reply (normal for EXPN) exactly one mailbox is to be specified on each line of the reply. In the case of an ambiguous request, for example, "VRFY Smith", where there are two Smith's the response must be "553 User ambiguous". The case of verifying a user name is straightforward as shown in examp 3.
The character string arguments of the VRFY and EXPN commands cannot be further restricted due to the variety of implementations of the user name and mailbox list concepts. On some systems it may be appropriate for the argument of the EXPN command to be a file name for a file containing a mailing list, but again there is a variety of file naming conventions in the Internet. The VRFY and EXPN commands are not included in the minimum implementation (
Section 4.5.1
), and are not required to work across relays when they are implemented.
3.4. SENDING AND MAILING
The main purpose of SMTP is to deliver messages to user's mailboxes. A very similar service provided by some hosts is to deliver messages to user's terminals (provided the user is active on the host). The delivery to the user's mailbox is called "mailing", the delivery to the user's terminal is called "sending". Because in many hosts the implementation of sending is nearly identical to the implementation of mailing these two functions are combined in SMTP. However the sending commands are not included in the required minimum implementation (
Section 4.5.1
). Users should have the ability to control the writing of messages on their terminals. Most hosts permit the users to accept or refuse such messages. The following three commands are defined to support the sending options. These are used in the mail transaction instead of the MAIL command and inform the receiver-SMTP of the special semantics of this transaction: SEND FROM:
The SEND command requires that the mail data be delivered to the user's terminal. If the user is not active (or not accepting terminal messages) on the host a 450 reply may returned to a RCPT command. The mail transaction is successful if the message is delivered the terminal.
SOML FROM: The Send or Mail command requires that the mail data be delivered to the user's terminal if the user is active (and accepting terminal messages) on the host. If the user is not active (or not accepting terminal messages) then the mail data is entered into the user's mailbox. The mail transaction is successful if the message is delivered either to the terminal or the mailbox. SAML FROM: The Send and Mail command requires that the mail data be delivered to the user's terminal if the user is active (and accepting terminal messages) on the host. In any case the mail data is entered into the user's mailbox. The mail transaction is successful if the message is delivered the mailbox. The same reply codes that are used for the MAIL commands are used for these commands.
About the Author
K.V.N.R.S.KRISHNA M.I.T,
LECTURER IN COMPUTER SCIENCE
SVRMCOLLEGE,
NAGARAM
CEELL:9394115353
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