Discuss about Simple Mail Transfer Protocol briefly.

SMTP:

It is sands for Simple Mail Transfer Protocol, is a protocol for sending e-mail messages among servers. Most e-mail systems which send mail over the Internet employ SMTP to send messages through one server to other; the messages can then be retrieved along with an email client by using either POP or IMAP.

SMTP (Simple Mail Transfer Protocol) is a relatively simple, text-based protocol, here one or more recipients of a message are given (and within most cases verified to exist) and after that the message text is transferred.
Commands of Simple Mail Transfer Protocol are generate by the sender-SMTP and also sent to the receiver-SMTP.

It replies are sent through the receiver-SMTP to the sender-SMTP into response to the commands. If a direct connection does not exist among the sender and the last destination, the message may be sent through one or more relay SMTP-servers. This relay SMTP-servers first functions as receivers and after that relays the message to the next SMTP. To be capable to give the relay ability the SMTP-server should be supplied along with the name of the ultimate destination host and also the destination mailbox name.