INTRODUCITON TO SMTP
SMTP stands for Simple Mail Transfer Protocol. It is a TCP/IP protocol that specifies how computers exchange electronic mail. It works with post office protocol (POP).
SMTP is used to upload mail directly from the client to an intermediate host, but only computers constantly connected such as Internet Service Providers (ISP) to the Internet can use SMTP to receive mail. The ISP servers then offload the mail to the users to whom they provide the Internet service.
SMTP uses TCP port number 25 for his service. Therefore e-mail is delivered from source to destination by having the source machine established a TCP connection to port 25 of the destination machine.
To send a mail, a system must have a client MTA, and to receive a mail, a system must have a server MTA. SMTP transfers this message from client MTA to server MTA. SMTP uses commands and responses to transfer the message between an MTA client and MTA server In order to send a mail, SMTP is used two times: one between the sender and the sender’s mail server, and the other between the two mail servers.