Simple Mail Transfer Protocol (SMTP) is a set of basic communication guidelines that allow for software to transmit emails throughout the internet.
Email software is mostly designed to use SMTP for communication reasons when transmitting email, and for this purpose it only works for outgoing transmissions. When email programs are set up they usually will have to submit the address of their internet service provider’s SMTP server for outgoing transmissions.
Generally speaking, there are two other main protocols; POP3 and IMAP. They are used for the retrieval and storage of a transmission. SMTP establishes a set of codes that greatly simplify the communication of email messages between servers.
It is essentially a type of shorthand that allows a server to categorize parts of a transmission into categories that other servers can understand. Any email message has a sender, a recipient (or multiple), a message body, and commonly a title heading.
From the perspective of users, when they write an email message, they experience the user-friendly interface of their email software, however, once that message goes out on the Internet, everything is encoded into strings of text.
This text is separated by code words or numbers that identify the purpose of each section. SMTP provides those codes, and email server software is designed to understand what they mean. The other main reason-to-be of SMTP is to set up rules of communication between servers.