Wikipedia
The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission As an Internet standard, SMTP was first defined in 1982 by RFC 821, and updated in 2008 by RFC 5321 to Extended SMTP additions, which is the protocol variety in widespread use today Mail servers and other message transfer agents use SMTP to send and receive mail messages Proprietary systems such as Microsoft Exchange and IBM Notes and webmail systems such as Outlookcom, Gmail and Yahoo Mail may use non-standard protocols internally, but all use SMTP when sending to or receiving email from outside their own systems
Simple Mail Transfer Protocol - Wikipedia, the free encyclopedia