Simple Mail Transfer Protocol (SMTP)
The SMTP protocol defines how mail messages are sent between hosts. SMTP is a connection-oriented protocol; it uses TCP connections to guarantee error-free delivery of messages. SMTP is not overly sophisticated and requires that the destination host always be available. For this reason, mail systems spool incoming mail so that users can read it at a later time. How the user then reads the mail depends on how the client accesses the SMTP server. SMTP is an application layer protocol. Today, SMTP is often used to send email between servers, whereas another protocol such as POP3 or IMAP4 is used to download the email from the server to a client system.