SMTP Commands
Simple Mail Transfer Protocol (SMTP) is an ASCII protocol. It is based on client-server model. It uses TCP port number 25 for this service. Therefore e-mail; is delivered from source to destination by having the source machine established a TCP to port 25 of the destination machine. To send mail, a system must contain client MTA and to receive a mail, a system must have server MTA. SMTP transfers this message from client MTA to the server MTA. To send a mail, SMTP is used two times: one between sender and sender’s mail server and the other between the two mail servers.
SMTP Commands :
These commands are sent from the client to the server. Each command consists of a keyword followed by zero or more arguments. It means there are also some keywords that do not contain any argument. The format of the command is as follows :
Keywords : argument(s)
There are 14 different SMTP commands which are given in the following table :
| S.No. | Keyword | Command form | Description | Usage |
|---|---|---|---|---|
| 1. | HELO | HELO<SP><domain><CRLF> | It provides the identification of the sender i.e. the host name. | Mandatory |
| 2. | MAIL<SP>FROM : <reverse-path><CRLF> | It specifies the originator of the mail. | Mandatory | |
| 3. | RCPT | RCPT<SP>TO : <forward-path><CRLF> | It specifies the recipient of mail. | Mandatory |
| 4. | DATA | DATA<CRLF> | It specifies the beginning of the mail. | Mandatory |
| 5. | QUIT | QUIT<CRLF> | It closes the TCP connection. | Mandatory |
| 6. | RSET | RSET<CRLF> | It aborts the current mail transaction but the TCP connection remains open. | Highly recommended |
| 7. | VRFY | VRFY<SP><string><CRLF> | It is use to confirm or verify the user name. | Highly recommended |
| 8. | NOOP | NOOP<CRLF> | No operation | Highly recommended |
| 9. | TURN | TURN<CRLF> | It reverses the role of sender and receiver. | Seldom used |
| 10. | EXPN | EXPN<SP><string><CRLF> | It specifies the mailing list to be expanded. | Seldom used |
| 11. | HELP | HELP<SP><string><CRLF> | It send some specific documentation to the system. | Seldom used |
| 12. | SEND | SEND<SP>FROM : <reverse-path><CRLF> | It send mail to the terminal. | Seldom used |
| 13. | SOML | SOML<SP>FROM : <reverse-path><CRLF> | It send mail to the terminal if possible; otherwise to mailbox. | Seldom used |
| 14. | SAML | SAML<SP>FROM : <reverse-path><CRLF> | It send mail to the terminal and mailbox. | Seldom used |
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