SMTP is short for Simple Mail Transfer Protocol, and is an email protocol which is still in widespread use today. While SMTP can be used for sending and receiving email messages, it is mostly used to send and relay email messages between mail servers. SMTP was first defined in RFC 821; this RFC was published in 1982, and cemented SMTP as a pillar of electronic mail technology.
As the name suggests, one drawback with SMTP is that it is "simple" and does not feature the ability to handle complex email messages. While it was once adequate for handling many of the needs of an email client, that is no longer the case. As already stated, SMTP's primary role is in sending and relaying email messages between servers; the POP and IMAP protocols are generally used to retrieve email messages from a server, while MIME handles attachments and additional message formatting/encoding.
SMTP applications typically use TCP port 25 (sometimes TCP port 587) when sending messages to a mail server; email protocols (which have other roles) do not use this port, for example: POP typically uses TCP port 110; IMAP typically uses TCP port 143. SMTP transfers usually consists of three parts: 1) a command for a return mail address (if the message cannot be sent); 2) a command to issue a receipt for the message; 3) a command for the data of the message (header and message content).