1. Chapter 30 Electronic Mail Representation & Transfer

2. 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 (eg.
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.

3. Message Format
An 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 software are passed through unchanged => good for communication between applications.

4. Multipurpose Internet Mail Extensions
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.

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

6. Mail Transfer & SMTP
user interacts with interface program when writing or reading messages.
The underlying 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 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.

7. 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 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.

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

9. 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 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 address from a particular mailing list

10. Mail Gateways Aka mail relay (fig 30.5)
a computer used to forward .
mail gateway can be used to make 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.

11. 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

12. Mail Transfer at CSLA
(mail.jpg)