2.5 Electronic Mail
E- mail, short for electronic mail, is a system in which a computer user can exchange messages with other computer users using a communications network. To use e-mail, you must have access to a computer that is linked to the outside world, via a modem, phone line or fiber optic cable(Ethernet).
The diagram above shows a typical sequence of events that takes place when Alice composes a message using her mail user agent (MUA). She types in, or selects from an address book, the e-mail address or her correspondent. She hits the “send” button.
- In the first stage the Alice’s message in Internet e-mail format uses the Simple Mail Transfer Protocol (SMTP) to pass the message to the local server.
- The (Message Transfer Agent ) MTP looks at the destination address provided in the SMTP protocol, in this case firstname.lastname@example.org. The part before the @ sign is the local part of the address, often the username of the recipient, and the part after the @ sign is a domain name. The MTA looks exchange servers accepting messages for that domain.
- The DNS server for the b.org domain, is ns,b.org, responds with an MX record listing the mail exchange servers for that domain, in this case mx.b.org,a server run by Bob’s ISP.
- Smtp.a.org sends the message to mx.b.org using SMTP, which delivers it to the mailbox of the user bob.
- Bob presses the “get mail” button in his MUA, which picks up the message using the POP or IMAP Protocol