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How emails are sent [the action of the SMTP servers]

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 : Message sending to server

The SMTP server should next break up the email to two different parts: the recipient name (Brendon) and the domain name (pqr.com).


Fig 3 : Resolution of recipient name and host name

Then the server should check the domain name whether it is of the same network (as of John's) or of a different network. If it is of the same network, it is just a simple message between two different nodes in the same network. If the sender's and receiver's domains are different, then the SMTP server should know to which IP address the email should be sent.



Fig 4 : Finding the appropriate domain


For this purpose, the SMTP server should communicate with the DNS (Domain Name Server) to obtain the IP address of pqr.com. Then the SMTP server just sends the message to Network B using the IP address obtained from the DNS.



Fig 5 : DNS providing the IP address of the host
Once the IP address is found, the message could be sent from Network A to Network B using an internet connection as follows:


Fig 6 : Sending the email over an internet connection

The server at Network B recognizes that the message's target is to its network and then again breaks up the email to find the recipient name. Finally it uses its POP3 server to hand in the message to Brendon's node.



Fig 7 : Using POP3 server to send the message to Brendon's node


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