SMTP
Simple Mail Transfer Protocol, a protocol for sending e-mail messages among servers. Most e-mail systems that send mail over the Internet use SMTP to send messages from single server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. SMTP is a relatively easy, text-based protocol, where one or more recipients of a message are specified (and in most cases verified to exist) and then the message text is transmitted.