SMTP is an acronym for Simple Mail Transfer Protocol. This is an Internet standard, specified in RFC-821, and as its name implies, is a protocol for transferring mail messages. When an SMTP server receives a piece of mail, it does one of two things: forwards the email to a host closer to the intended recipient, or if the recipient is local, places the email in the recipient’s mailbox. Thus, SMTP provides a technique for putting messages in a mailbox, but it doesn’t define a technique for retrieving existing messages from a mailbox. To this end, the Post Office Protocol, Version 3 (POP3) has been designed, as specified in RFC-1725. Its explicit purpose is to allow remote access to a mailbox managed on a remote computer.
In practice, this means that SMTP can send Internet email, and POP3 can retrieve Internet email.
As is common for Internet protocols, both mail protocols use a simple conversation between a client and a server. This conversation is "line-based” (meaning all commands and responses are sent as complete lines) and works exclusively with 7-bit ASCII data. Each protocol defines its own special command and response sequence to support its various options.
The mail messages handled by both these protocols must be formatted as specified in various RFCs, starting with RFC-822, to the latest, which is RFC-1521. In a nutshell, these RFCs define the format of the message header (a list of headers for the message, including the subject, recipient information, etc.), ...