How an email is sent from one computer to the other?

Three steps are now required to deliver the message to its final destination, sender to local mail server, local mail server to destination server and destination server to recipient. If the receiver's mail server is not functioning then the sender's server retains the message and tries to deliver it later. Mail servers are normally provided and maintained by Internet Service Providers (ISPs) - e.g. AOL.

SMTP is used to transfer the message from the sender local server and between the servers, but it cannot be used to retrieve the message by the recipient. The reason for this is that SMTP is a 'push' protocol (i.e. it is designed to deliver messages) whereas retrieval is a 'pull' operation. To solve this remaining problem, another protocol is required. Two popular mail access protocols are known as POP3 (Post Office Protocol - version 3) and IMAP (Internet Mail Access Protocol). Note however, SMTP is still required to send e-mail.

1. user A invokes his/her user agent for e-mail, provides e-mail address (e.g. usera@hotmail.com) compose and then sends the message via user agent
2. user A’s user agent sends message to his/her email server - placed in a message queue
3. The client side of SMTP opens a TCP connection to an SMTP server
4. After some initial SMTP handshaking, the SMTP client sends user A’s message into the TCP connection
5. At user B’s mail server host, the server side of SMTP receives the message – places the message in user B’s mailbox
6. user B invokes his/her user agent to read the message at his convenience