An application level protocol used in Linux distribution, which is responsible for the transfer of mail messages. Simple Mail Transfer Protocol, which is abbreviated as SMTP is an e-mail system protocol used for transferring mail messages between servers. It simply handles messages over TCP/IP networks. It consists of a mail user agent (MUA), mail transfer agent (MTA), and a mail delivery agent (MDA).The mail user agent reads, writes and sends messages. The mail transfer agent transfers those messages from one system to the other. Whereas, the mail delivery agent delivers the messages to the mailbox of a local recipient.