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 |
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 |
Last Updated :
13 Sep, 2021
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