Computer and Communication Networks (paperback)

9.4. Electronic Mail (E-mail)

9.4.1. Simple Mail Transfer Protocol (SMTP) and E-mail

The Simple Mail Transfer Protocol (SMTP) plays a major role in transferring Internet electronic mail. This protocol transfers electronic mail ( e-mail ) from the mail server of a source to the mail servers of destinations. SMTP is older than the Hypertext Transfer Protocol (HTTP), the Web communication protocol, and imposes certain restrictions, such as limits on the size of e-mail content.

In Figure 9.8, user 1 is in a residential area, has an Internet service provider (ISP), and is sending an e-mail to user 2, working in an organization. Suppose that the mail servers are isp.com and organization.com, respectively. Thus, user 1 and user 2 have e-mail addresses of user1@isp.com and user2@organization.com, respectively. The procedure for an e-mail exchange between user 1 and user 2 is as follows .

Begin SMTP Between Two Users

1. User 1 provides user 2's e-mail address (user2@organization.com) and composes its message.

2. User 1 sends the message to its mail server (isp.com).

3. Server isp.com places the message in its queue.

4. SMTP on user 1's mail server notices the message in the queue and opens a TCP connection with the organization mail server (organization.com).

5. Initial SMTP handshaking takes place between the two servers.

6. The message is sent to organization.com's mail server, using the established TCP connection.

7. User 2's mail server receives the message and then puts it in user 2's mailbox, ready to be retrieved by user 2.

A user mailbox is a space in the mail server allocated to the user to keep its e-mail. Also, SMTP is designed to connect only the two mail servers of the associated parties, regardless of the distance between the two users. Consequently, this protocol involves only the two mail servers of the communicating users.