VIII-Simple Mail Transfer Protocol (SMTP)

 

The basic objective of Simple Mail Transfer Protocol is to transfer mail

reliably and efficiently.

 

The SMTP design in based on the following model of communication: As

A result of a user mail request, the sender-SMTP establishes a two-way

transmission channel to a receiver-SMTP. The receiver-SMTP may be the

destination or just an intermediate. SMTP commands are generated by

the sender-SMTP and sent to the receiver-SMTP. SMTP replies are sent

form 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, 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 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, or one-at-atime

 

The SMTP provides mechanisms for the transmission of mail: directly form the sending

user’s hosts when the two hosts are connected to the same transport service, or via one or

more reply SMTP-servers when the source and destination hosts are not connected to the

same transport service.

 

To provide 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 sent 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.