Electronic Mail

E-Mail

• Client Software and Mail Hosts– Client PC has E-Mail client software that

communicates with user’s mail host– Mail hosts deliver outgoing mail to other mail

hosts

PC withE-Mail Client Mail Host Mail Host

PC withE-Mail Client

SMTP

• Simple Mail Transfer Protocol (SMTP)– Standard for mail host-mail host exchanges– E-Mail Client often sends messages to mail host

via SMTP, but not always

PC withE-Mail Client Mail Host Mail Host

SMTP SMTP

SMTP Operation

• For Each Message, the Sending Process– Makes a connection– Gives name of sender (From) and gets OK– Gives names of receivers individually and

gets OK for each separately– Asks to send message, gets OK– Sends message, gets confirmation– Closes connection

Receiving and Sending E-Mail

• User’s Mail Host Stores Incoming Files in the User’s Mailbox– User later retrieves them– User also sends outgoing mail

Client PC Mail HostWith User’s Mailbox

Receive Mail

Send Mail

File Server Program Access E-Mail

• Use proprietary ways to send messages, get messages, and in other ways interact with the mail host

– Can be used only on LANs

– Cannot be used over the Internet

LAN

PC with FSPAE-Mail Program

POP Clients• POP (Post Office Protocol) is the most

popular standard for mail downloading

– Download messages all or selectively

– Send outgoing messages via SMTP

– Works via Internet

PC withInternet E-Mail Client Mail Host Mail Host

SMTP SMTP

POP

POP Operation

• Several client-mail host interactions needed to download new mail– Log into mail host

– Can ask how many new messages there are and how long they are

– Can download all or download one at a time

– If download one at a time, can decide based on length

– Can delete messages on host after downloading

– Close the session

IMAP Clients

• IMAP (Internet Message Access Protocol)

– But not as widely supported as POP

– Send outgoing messages via SMTP

– Works via the Internet

PC withInternet E-Mail Client Mail Host Mail Host

SMTP SMTP

IMAP

IMAP Clients

• IMAP (Internet Message Access Protocol)– More sophisticated than POP

• Can do more on mail server’s mailbox than download and delete messages; can fully manage the mailbox

PC withInternet E-Mail Client Mail Host Mail Host

SMTP SMTP

IMAP

Browser Clients (Web-Based E-Mail)

• Client is Browser

• Mail Host is a Webserver– Mail host sends HTML pages to client

– User types messages and retrieval data in forms, sends back

• All communication is via HTTP

PC withBrowser

WebserverMail Host

Mail Host

HTTP SMTP

Telnet Clients• Some mail hosts support Telnet

• Telnet client on PC emulates a simple terminal

– No color or graphics

– Monospaced Text

– Sometimes only way to interact with a mail host

PC withTelnet Client

Mail HostSupporting Telnet

Mail Host

Telnet SMTP

Recap on Internet E-Mail Transmission

• Communication Between Mail Hosts– SMTP

• Communication From Client to its Mail Host

– SMTP

– Proprietary file server program access on LANs

– HTTP

– Telnet

Recap on Internet E-Mail Transmission

• Communication to Client from its Mail Host to deliver messages

– POP or IMAP

– Proprietary file server program access on LANs

– HTTP

– Telnet

Note on Internet E-Mail Transmission

• Client and Server can Communicate Over the Internet– Except for File Server Program Access– You can access your e-mail from anywhere– Must have the right client program

Client PC Mail HostWith User Mail Box

Message Structure Standards

• RFC 822– Text-only message bodies

• MIME– Multimedia message bodies and headers

– Not widely used for bodies or headers

• HTML Bodies– Becoming common

– Not well standardized; Limited interoperability between mail clients

Attachments

• Send a message– Attach a file (word processing document,

spreadsheet, graphic, etc.)– E-mail can be a file delivery mechanism

Attachments

• Viruses– Attachments may contain viruses

• Even messages without attachments may contain viruses today

– Virus scanning before opening is critical

Attachments

• Problem– Attached files use all 8 bits of each byte

• Called binary data

– On Internet, can only use the first seven bits• Called 7-bit ASCII

• In Internet transmission, 7th bit may be truncated if send binary file

1010101010101010

Binary

x1010101x1010101

Internet

Attachments• Internet Encoding

– Files must be Internet encoded before transmission to travel over the Internet using only the first 7 bits in each byte

– At the receiving end, files must be Internet decoded so that applications can read them

1010101010101010

Binary

x1010101x1010101

Internet

x1010101x1010101

Internet

1010101010101010

Binary

InternetEncoding

InternetTransmission

InternetDecoding

Attachments

• Internet Encoding Example (There are Other Internet Encoding Standards)– Break file into groups of three data bytes (24

bits)– Create group of four encoded bytes (32 bits)

1111111111111111 0000000000000000 1111111111111111Data Bytes

Encoded Bytes

Attachments

• Internet Encoding Example (There are Other Internet Encoding Standards)– Put six bits of each data byte in each outgoing

byte• Leaves two bits free in each outgoing byte

1111111111111111 0000000000000000 1111111111111111

xx111111xx111111 xx11xx11

Data Bytes

Encoded Bytes

6,2

Attachments

• Internet Encoding Example (There are Other Internet Encoding Standards)– Put six bits of each data byte in each outgoing

byte• Leaves two bits free in each outgoing byte

1111111111111111 0000000000000000

xx111111xx111111 xx110000xx110000 xx0000xx0000

Data Bytes

Encoded Bytes

4,4

Attachments

• Internet Encoding Example (There are Other Internet Encoding Standards)– Put six bits of each data byte in each outgoing

byte• Leaves two bits free in each outgoing byte

1111111111111111 0000000000000000 1111111111111111

xx111111xx111111 xx110000xx110000 xx000011xx000011 xx111111xx111111

Data Bytes

Encoded Bytes

2,6

Attachments• Internet Encoding Example (There are

Other Internet Encoding Standards)– Lowest 31 ASCII codes are control codes– Add 32 (100000) to each outgoing byte so

that it will not become a control code– 8th bit is still free, as required

xx111111xx111111 xx110000xx110000 xx000011xx000011 xx111111xx111111Encoded Bytes

x1011111x1011111 x1010000x1010000 xx100011xx100011 x1011111x1011111Encoded Bytes

Add 100000

Attachments• Internet Encoding Standards

– Communicating mail clients must use the same Internet encoding standard to encode and decode

– UUENCODE is common in UNIX – MIME

• Several versions of MIME exist• Basic MIME is almost universally supported by

e-mail clients today

– Binhex is commonly used on Macintoshes

Attachments• E-Mail users should negotiate before

sending an attachment– Internet encoding standard they will use– Application file format they will use

• If same application program and version, fine

• If same application program and different versions, send in format of older version

• If different application programs, send in a format and version the other can import

E-Mail Standards Recap

• Transmission Standards– Sending messages (SMTP, etc.)– Receiving messages (POP, IMAP, etc.)

• Message Structure Standards– Message header and body (RFC 822, MIME,

HTML)– Attachments: common Internet encoding

standard– Attachments: common application file format