Simple Mail Transfer Protocol Explained – Everything You Need To Know

6 Facts about Simple Mail Transfer Protocol

  • Simple Mail Transfer Protocol, or SMTP, was one of the first commercially available protocols that determined how electronic messages – including Email and files – were sent. 
  • Sendmail was one of the first companies to utilize SMTP. 
  • There are two types of SMTPs: One involves sending the content directly (end-to-end), and one involves storing it for later sending (store-and-forward). End-to-end is used for communication outside of networks, while store-and-forward is used within networks. 
  • Despite the fact that SMTP is more than thirty years old, it is still a wildly popular protocol that is used when sending Emails.
  • However, Simple Mail Transfer Protocol is not the only Email protocol that is used today. Other examples include IMAP and POP3.
  • Simple Mail Transfer Protocol can be used for sending any electronic data on the internet, including texts, pictures, videos, links, and more.

Simple Mail Transfer Protocol History

STMP was originally used to replace outdated electronic exchange protocols. These protocols were necessary in order to accommodate different electronic and internet systems that was evolving as ARPANET moved beyond government controls and into the private sector.

Jon Postel first developed the Simple Mail Transfer Protocol in 1980, and the SMTP replaced the File Transfer Protocol around that time. It was allocated port number 57 for use around that time period, and its widespread adaption quickly began. Electronic mail agents began to appear during that time – for example, Sendmail, one of the first Email services, appeared in 1982 and used SMTP. Security concerns eventually made SMTP out of date, but the system evolved into other versions, such as EMSTP and POP3. 

Simple Mail Transfer Protocol: How It Works

A modern flow chart can easily be understood to determine how SMTP works. For example: 

  • A user is at their computer and using any number of Email services, such as Gmail or AOL. They type an Email and click the send button.
  • The Email enters the user’s send queue on their own server. If it is a normal personal commercial user, the queue is likely not particularly long, and the Email is sent quickly.
  • The Email arrives at a Message Transfer Agent, which makes a connection between the Message Transfer Agent of the intended recipient. This occurs via SMTP protocol, which connects those two Message Transfer Agents. It will usually occur on port number 25. 
  • From there, the Email travels to the intended recipient’s Email inbox, where it can be accessed after it has been downloaded by the recipient’s server.

Simple Mail Transfer Protocol: Historical Significance

The importance of the development of SMTP cannot be understated, as it was critical to facilitating the advancement of electronic communication between computers. The fact that it is still used today – in largely the same form – also speaks to how useful and powerful the invention of SMTP has been.

That being said, it is important to realize that SMTP was originally designed largely for internal communication between networks and that the protocol had to be expanded and altered in order to make it robust and effective when communicating with other networks. That resulted in the ESMTP, or Extended Simple Mail Transfer Protocol, that first came into existence in 1995.

The success of the Simple Mail Transfer Protocol set the stage for the mass adoption of Email clients. For example, Gmail, Microsoft Outlook, and many additional services simply would not have been commercially viable were it not for the success of SMTP. 

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