Online SMTP Response

SMTP = Simple Mail Transfer Protocol.SMTP is a relatively simple, 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 transferred. It is a client-server protocol, where the client transmits an email message to the server. Either an end-user's email client, a.k.a. MUA (Mail User Agent), or a relaying server's MTA (Mail Transfer Agents) can act as an SMTP client.
Source: WikiPedia