SMTP means Simple Mail Transfer Protocol
SMTP is the global standard for sending e-mail over the Internet. It is a relatively simple protocol: SMTP first mentions the sender of the message, then 1 (or multiple) recipients, and then the shipping information and content. SMTP initiates the sender's initiative, so it is desirable that the receiver is always waiting. In the past, not all computers were permanently connected to the Internet (nowadays it is increasingly common), and SMTP is not the best way to send a message. Therefore, these messages are stored with the ISP until the recipient is logged in again with POP3 or IMAP (to retrieve the e-mail messages). A major limitation of the original SMTP protocol is that no reliable way to check if the sender is really who he claims to be. This makes it possible to send spam.
How Email is Sent and received on The Internet [YouTube] G6NMFHuef6Y [/youtube]
If you want to read more about SMTP, order a book at BOL.