SMTP or simple mail transfer protocol is an internet standard for email. What this means is that it is a normative arrangement of a methodology or technology related to the internet that was created for the purpose of sending emails.
In the beginning, different kinds of one to one electronic messaging were employed in the 1960s. People conversed with each other using systems developed for particular mainframe computers. Meaning, a person who is using Windows-based computer could only email a person who was also using a Window-based computer and not one who was using a Macintosh. As more and more computers started to get interconnected, standards were developed to let users of diverse systems to email each other. Here is how this works.