What is SMTP?
SMTP or Simple Mail Transfer Protocol is a way to transfer email reliably and efficiently. 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. You can think of SMTP as the language that mail servers use to communicate among themselves.

SMTP is used to send mail to the recipient's mailbox. The recipient may use various methods to access the emails in his mailbox. A couple of methods that are very popular are POP3 and IMAP. These protocols allow a user to access his messages stored on a remote mail server.

Basics of SMTP
When the user wants to send a message to someone, the sender-SMTP establishes a two-way transmission channel to a receiver-SMTP. SMTP commands are generated by the sender-SMTP and sent to the receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the sender-SMTP in response to the commands. In case a direct connection does not exist between the sender and the final destination, the message may be sent via one or more relay SMTP-servers. The relay SMTP-servers first act as receivers and then relays the message to the next SMTP. To be able to provide the relay capability the SMTP-server must be supplied with the name of the ultimate destination host as well as the destination mailbox name.

Once the transmission channel is established, the SMTP-sender sends a MAIL command indicating the sender of the mail. If the SMTP-receiver can accept mail it responds with an OK reply. The SMTP-sender then sends a RCPT command identifying a recipient of the mail. If the SMTP-receiver can accept mail for that recipient it responds with an OK reply; if not, it responds with a reply rejecting that recipient (but not the whole mail transaction). The SMTP-sender and SMTP-receiver may negotiate several recipients. When the recipients have been negotiated the SMTP-sender sends the mail data, terminating with a special sequence. If the SMTP-receiver successfully processes the mail data it responds with an OK reply. The dialog is purposely lock-step, one-at-a-time.