The mail system has two major parts : MTAs and UAs. A message transfer agent (MTA) routes a mail message towards its final destination by sending the message to another MTA. A user agent (UA) interacts with an end-user and allows the user to send and receive mail messages.
On the Internet, MTAs communicate with each other using the Simple Mail Transfer Protocol (SMTP). SMTP follows the client/ server model of computing. The SMTP client initiates the dialogue with an SMTP server by sending a message to the remote server. This message includes the following information : the sender address, the recipient address (maybe more than one), and the message itself.
Another useful mail protocol is Post Office Protocol (POP). With POP one can set up a machine as a mail drop point. POP lets a userís mailbox reside on a remote host, but allows the user to retrieve the messages from the remote mailbox on demand. As such, POP is user driven, since mail is not transferred until the user requests that be done. After the transfer, the user can read mail on the local system whenever itís convenient. Replies are relayed to the POP server using SMTP. There are two versions of POP -- POP2 and POP3.
The SMTP protocol suffers from one significant defect : it understands only 7-bit ASCII characters. Consequently, you canít use SMTP to send binary data such as graphics or programs. Multipurpose Internet Mail Extensions (MIME) provides a standard by specifying a set of encoding rules and header extensions to the Internet standard message specification. These rules let a UA encode messages with text, graphics, and even sound into 7-bit ASCII.
Titany answered the question on November 30, 2021 at 08:33
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