Chapter 30 Electronic Mail Representation & Transfer

Electronic Mailboxes and Addresses Mailbox is a location used to hold mail (eg. jupiter:/var/spool/mail/mailboxid is used to hold incoming mail) mailbox address format mailboxID@computer (eg. username@domainname) Each electronic mailbox has a unique address, which is divided into two parts: the first part (mailbox identifier) is interpreted locally for selecting a particular mailbox into which a message should be placed second part is used to select a destination computer.

Email Message Format An email message consists of two parts separated by a blank line. The first part is the header which specifies the sender, recipients , and subject. The second part is the body that contains the text of the message. Each header line begins with a keyword (eg. To, From, Cc, Bcc, Date, Subject) followed by a colon and additional information (fig 30.2). Header lines which are not understood by email software are passed through unchanged => good for communication between email applications.

Multipurpose Internet Mail Extensions Email systems originally designed use ASCII text only binary data (eg. graphics and programs) has to be encoded into text via hex representation Base64 encodes every 6 bits into one of 64 ASCII characters MIME (multipurpose Internet mail extensions) standard allows a sender to encode non-text data for transmission. MIME does not specify a single standard for encoding; sender can inform a recipient about the encoding used. message header lines used to indicate that the message adheres to MIME format MIME-Version: 1.0 Content-Type: text/plain (or Multipart/Mixed, audio, video, app) data type and the encoding specified in body. This allows the message body to contain different sections with different encoding.

Email and Application Programs An email address can correspond to a program which gets invoked automatically at destination instead of a mailbox. This facilitates automated response email systems

Mail Transfer & SMTP user interacts with email interface program when writing or reading messages. The underlying email system contains a separate mail transfer program that handles the details of sending a copy of the message to a remote computer (fig 30.3). Outgoing messages are placed by the email interface program to a queue that the mail transfer program handles. (eg. jupiter: /var/spool/mqueue) The mail transfer program waits for a message to be placed on its queue, and then transfers a copy of the message to each recipient, which can be on local computer or on remote computer.

Mail Transfer & SMTP (cont.) To send mail to remote computer, the mail transfer program running as a client in the background on sender’s computer contacts the email server on the recipient’s computer by opening a TCP connection to port 25. Simple Mail Transfer Protocol (SMTP) is then used by the two programs which allows the sender to identify itself, specify a recipient, and to transfer the message to the recipient’s mailbox. SMTP guarantees reliable delivery: sender does not delete its copy of message until receiver has stored a copy on its hard disk.

SMTP Protocol RFC 821 SMTP commands HELO MAIL FROM: sender email address RCPT TO: recipient email address DATA QUIT

Mail Exploders, Forwarders, and Mailing Lists Mail exploder, mail forwarder, or mailing list is a program that can forward copies of a message. An exploder uses a database(eg. flat file of list name followed by email address of users in mailing list) to provide communication among large groups of participants (fig 30.4). When a message is sent to a mailing list, the exploder forwards a copy to each member of the list. Automated list manager is an automated program that accepts a request to add (eg. subscribe mailbox listname) or remove a user’s email address from a particular mailing list

Mail Gateways Aka mail relay (fig 30.5) a computer used to forward email. mail gateway can be used to make email addresses of all users uniform. A database on gateway must contain an entry for user that specifies the employee’s mailbox on a specific machine in the company. Exploder on gateway would forward mail to user’s internal mailbox. advantage of mail gateways is flexibility in moving internal mailboxes to different servers.

Mailbox Access POP(Post Office Protocol) IMAP POP client run on microcomputers to access a mailbox on a remote computer. POP server access the contents of user’s mailbox on remote server. Mail server uses SMTP protocol while POP server uses POP protocol (fig 30.6). IMAP IMAP client software runs on microcomputer IMAP server runs on mail server

Mail Transfer at CSLA (mail.jpg)