Electronic Mail EMail Client Software and Mail Hosts

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 with E-Mail Client Mail Host PC with E-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 SMTP PC with E-Mail Client SMTP Mail Host

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 Receive Mail Send Mail Client PC Mail Host With User’s Mailbox

File Server Program Access EMail • 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 PC with FSPA E-Mail Program LAN

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 SMTP POP PC with Internet E-Mail Client Mail Host

POP Operation • Several client-mail host interactions needed to download new mail – Log into mail host – Can ask how many new messages there 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 SMTP IMAP PC with Internet E-Mail Client Mail Host

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 SMTP IMAP PC with Internet E-Mail Client Mail Host

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 with Browser SMTP Webserver Mail Host

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 Telnet PC with Telnet Client SMTP Mail Host Supporting Telnet Mail Host

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 Host With 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, 7 th bit may be truncated if send binary file 1010 x 1010101 Binary 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 Internet Encoding Internet Transmission Internet Decoding 1010 x 1010101 1010 Binary Internet Binary

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) Data Bytes Encoded Bytes 1111 0000 1111

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 6, 2 Data Bytes 1111 0000 Encoded Bytes xx 111111 xx 11 1111

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 4, 4 Data Bytes 1111 0000 Encoded Bytes xx 111111 xx 110000 xx 0000

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 2, 6 Data Bytes 1111 0000 1111 Encoded Bytes xx 111111 xx 110000 xx 000011 xx 111111

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 – 8 th bit is still free, as required Encoded Bytes xx 111111 xx 110000 xx 000011 xx 111111 x 1010000 xx 100011 x 1011111 Add 100000 Encoded Bytes

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 email 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