Overview of Email protocols.
Electronic mail was one of the first applications of the fledgling Internet back in the 70ies of the last century. Mail represents basically an electronic equivalent of telegrams that can be sent without having a direct and simultaneous network connection with the recipient.
In email systems, a mail transfer agent (MTA) takes care of the message to be sent and repeatedly tries to deliver the message to the ultimate receiver until successful.
One of the first and still prevalent email protocols is SMTP (Simple Mail Transfer Protocol). It is a very simple protocol for sending electronic messages consisting of a header and a body between a sender and a recipient (relay agent or final mail transfer agent).
SMPT is not suited for receiving mail. Additional protocols like POP (Post Office Protocol) or the newer IMAP (Internet Mail Access Protocol) are needed to retrieve mail messages from a mail box.
SMPT was originally restricted to the ASCII character set so binary content or non-ASCII characters could not be used in mail messages. To overcome this limitation, MIME (Multipurpose Internet Mail Extensions) was devised to allow mail senders to encode non-ASCII content with special character encodings.