What is SMTP? SMTP stands for Simple Mail Transfer Protocol and is used by email carriers to send and receive emails. SMTP uses the store and forward technique to send emails between two connections. [Actually, there are two different servers running on a server machine : the SMTP server and the POP3 or IMAP server. We will focus only on the SMTP and the functionality of the SMTP server in this blogpost]. How does SMTP work [or how emails use SMTP?] ? Consider the following expected communication between John and Brendon: Fig 1 : Expected communication between John and Brendon Suppose John is going to send his email using Outlook express. First, Outlook express should connect to John's network and then to the SMTP server using Port 25 (SMTP usually uses TCP port 25 for connections). Then Outlook express should notify the server the message to be sent, the address of the sender and that of the recipient. Suppose Brendon's address is Brendon@pqr.com. Fig 2
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