Essential Concepts - Network Protocols (SMTP)

So far we have seen Protocols for website communication and file transfer along with security layer. We also use many other services over Internet no-a-days. Take for an example the E-mail service. So today, we are going to study the protocols used in sending and receiving E-mails.

SMTP (Simple Mail transfer Protocol) :

SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 or IMAP, that let the user save messages in a server mailbox and download them periodically from the server.

Due to its limitation of queuing the mails at receiving end, we can say that :

• The Simple Mail Transfer Protocol provides a standard for sending (not receiving) e-mails from one server to another.

• In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail.

IMAP (Internet Message Access Protocol) and POP3 (Post Office Protocol 3) :

IMAP (Internet Message Access Protocol) is a standard email protocol that stores email messages on a mail server, but allows the end user to view and manipulate the messages as though they were not stored locally on the end user's computing device(s). This allows users to organize messages into folders, have multiple client applications know which messages have been read, flag messages for urgency or follow-up and save draft messages on the server.

IMAP can be contrasted with another client/server email protocol, Post Office Protocol 3 (POP3). With POP3, mail is saved for the end user in a single mailbox on the server and moved to the end user's device when the mail client opens, i.e. when we open the mailbox, email automatically get downloaded and saved in our device. While POP3 can be thought of as a "store-and-forward" service, IMAP can be thought of as a remote file server (not saved on device but can be accessed online).

Most implementations of IMAP support multiple logins; this allows the end user to simultaneously connect to the email server with different devices. For example, the end user could connect to the mail server with his Gmail app and his Gmail desktop client at the same time. The details for how to handle multiple connections are not specified by the protocol but are instead left to the developers of the mail client.

NOTE : SMTP along with SSL is known as SMPTS.