With the advent of modern technology, communicating with people of different parts of the world has become very easy. With a click of a mouse, you can send your mail electronically to the person who may live in the other part of the world within seconds. Whenever you send a piece of e-mail, the e-mail client then interacts with the SMTP servers which handles the sending. In most of the e-mail systems that send mail over the internet use SMTP to send messages from one server to another and the messages can then be retrieved with an email client using either POP or IMAP.
In earlier days, the mode of communication was in various forms of one to one electronic messaging services. As all the computers were interconnected , the need to allow users of different systems to e-mail each other , was the need of the hour . In that way, the SMTP mail server was introduced in the early seventies. As the server is ASCII based, it do not deal well with binary files.
SMTP is a text based protocol, in which a mail sender communicates with a mail receiver by using some command strings and in this way, it supplies the necessary data over a reliable ordered data stream channel, typically a Transmission Control Protocol (TCP). SMTP servers were typically used internally to an organization, which is used in receiving mail for the organization from the outside and relaying messages from the organization to the outside.
All the communications that occurs on the internet is through the client-server connectivity. In other words, the client is the computer that connects through a unique internet protocol address through the internet and the server, such as SMTP servers, is another computer which is connected to the internet that takes care of hosting and serving web pages. These servers are nothing but computer programs that run on high speed machines which are designed in the manner that they handle all the requests of millions of clients. All the requests are served at the same time to those who access an email or web page.