For the vast majority of people right now, the real e-mail system consist of two different servers running on the main e-mail server machine. One is called the SMTP Server (Simple mail transfer protocol) which handles outgoing mail and the other is POP3 Server (Post Office Protocol) which takes the responsibility of incoming mail. The SMTP server listens on well-known port number 25, while POP3 listens on port 110.

Sending the mail (Role of SMTP server)

Whenever you send a piece of e-mail, your e-mail client interacts with the SMTP server to handle the sending.

Lets assume that i want to send a piece of e-mail. My e-mail ID is admin and having account on I want to send e-mail to manekari[at] and i am using standalone e-mail client like Windows live mail or Outlook Express. lets see what happens when i compose a message and press the send button.

  • E-mail client connects to the SMTP server at using port 25.
  • E-mail client has a conversation with the SMTP server, telling the SMTP server the address of the sender and the address of the recipient, as well as the body of the message.
  • The SMTP server takes the “to” address ( and breaks into two parts :
    • The recipient name (manekari)
    • The domain name (
      (If the “to” had been another user at, the SMTP server would simply hand the message to POP3 server for ) Since the recipent is at another domain, SMTP needs to communicate with the domain.
  • The SMTP server has a conversation with a Domain Name Server, or DNS. It says, “Can you give me the IP address of the SMTp server for” The DNS replies with the one or more IP address for the SMTP server(s) that operates.
  • The SMTP server at connects with the SMTP server at using port 25. It has the same simple text conversation that my e-mail client had with the SMTP server for WinSpark, and gives the message to the server. The server recognizes that the domain name for manekari is at, so it hands the message to yahoo POP3 server, which puts the message in manekari’s mailbox.

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