26.1 Electronic Mail Sending/Receiving Mail Addresses User Agent MIME Mail Transfer Agent Mail Access Protocols

Figure 26.1 Format of an

Figure address

Figure 26.3 User agent

Some examples of command-driven user agents are mail, pine, and elm. Note:

Some examples of GUI-based user agents are Eudora, Outlook, and Netscape. Note:

Figure 26.4 MIME multipurpose internet mail extensions

Figure 26.5 MIME header

Table 26.1Data types and subtypes in MIME TypeSubtypeDescription TextPlainUnformatted text Multiport MixedBody contains ordered parts of different data types ParallelSame as above, but no order DigestSimilar to mixed, but the default is message/RFC822 AlternativeParts are different versions of the same message Message RFC822Body is an encapsulated message PartialBody is a fragment of a bigger message Ext. BodyBody is a reference to another message Image JPEGImage is in JPEG GIFVideo is in GIF format VideoMPEGVideo is in MPEG format AudioBasicSingle-channel encoding of voice at 8 KHz Application PostScriptAdobe PostScript Octet-StreamGeneral binary data (8-bit bytes)

Figure 26.8 MTA client and server

Figure 26.9 Commands and responses

Figure delivery

Three major components:  User Agents  Mail Servers  Simple Mail Transfer Protocol: SMTP User Agent  a.k.a. “mail reader”  Composing, editing, reading mail messages  e.g. Outlook, Eudora, Thunderbird  Outgoing, incoming messages stored on server

 Mail Servers Mailbox contains incoming messages for user Message queue of outgoing (to be sent) mail messages  SMTP protocol between mail servers to send messages Client: sending mail server Server: receiving mail server

 SMTP (Simple Mail Transfer Protocol) is the procedure by which data packets are transferred from one networked machine to another.  SMTP defines the message format and the message transfer agent (MTA), which stores and forwards the mail.  SMTP is a relatively simple, text-based protocol, where one or more recipients of a message are specified and then the message text is transferred.

 Transfer between mail servers reliably and efficiently.  In order to send , the client sends the message to an outgoing mail server, which in turn contacts the destination mail server for delivery.  For this reason, it is necessary to specify an SMTP server when configuring an client.

 SMTP uses TCP port 25.  SMTP requires message (header & body) to be in 7 -bit ASCII  SMTP server uses CRLF.CRLF to determine end of message  No sender authentication.  Unsecured against spam.

 Mail client is configured with the name of a local mail gateway (SMTP server)  Mail client does not have to know how to deliver mail to everywhere

Message Format Header lines, e.g.,  To:  From:  Subject:  Different from SMTP commands! Body  The “message”, ASCII characters only message format: ♦ header lines, e.g., To: From: Subject: different from smtp commands! ♦ body the “message”, ASCII characters only message format: ♦ header lines, e.g., To: From: Subject: different from smtp commands! ♦ body the “message”, ASCII characters only

 HELLO: Sent by a client to identify itself, usually with a domain name  MAIL FROM: Identifies the sender of the message; used in the form MAIL FROM:  RCPT TO: Identifies the message recipients; used in the form RCPT TO:  DATA: Sent by a client to initiate the transfer of message content  QUIT: Terminates the session

TCP/IP Protocol Suite23 Figure Message transfer

 Replies to SMTP commands serve to ensure the synchronization of requests and actions in the process of mail transfer and to guarantee that the SMTP client always knows the state of the SMTP server.  Every command must generate exactly one reply.  An SMTP reply consists of a three digit number followed by some text. The number is for use by automata to determine what state to enter next; the text is for the human user.  Formally, a reply is defined to be the sequence: a three-digit code,, one line of text, and, or a multi-line reply.

REPLY CODESMEANING 211 System status, or system help reply 214 Help message 220 Service ready 221 Service closing transmission channel 250 Requested mail action okay, completed 354 Start mail input; end with. 421 Service not available, closing transmission channel

REPLY CODESMEANING 450 Requested mail action not taken: mailbox unavailable 451 Requested action aborted: local error in processing 500 Syntax error, command unrecognized 501 Syntax error in parameters or arguments 503 Bad sequence of commands 550 Requested action not taken: mailbox unavailable 551 User not local; please try 554 Transaction failed

This SMTP example shows how mail is sent by Smith at host Alpha.ARPA, to Jones and Green at host Beta.ARPA S: MAIL R: 250 OK S: RCPT R: 250 OK S: RCPT R: 550 No such user here S: RCPT R: 250 OK S: DATA R: 354 Start mail input; end with. S: Blah blah blah... S:...etc. etc. etc. S:. R: 250 OK

IMAP is an Internet Mail Access Protocol. It is a method of accessing electronic mail messages that are kept on a possibly shared mail server. In other words, it permits a "client" program to access remote message stores as if they were local. For example, stored on an IMAP server can be manipulated from a desktop computer at home, a workstation at the office, and a notebook computer while travelling, without the need to transfer messages or files back and forth between these computers. IMAP uses TCP/IP port 143.

 Short for Post Office Protocol, a protocol used to retrieve from a mail server. Most applications (sometimes called an client) use the POP protocol, although some can use the newer IMAP (Internet Mail Access Protocol).protocol server clientIMAP (Internet Mail Access Protocol)  POP3 uses TCP/IP port 110.

 With IMAP, all your mail stays on the server in multiple folders, some of which you have created. This enables you to connect to any computer and see all your mail and mail folders. In general, IMAP is great if you have a dedicated connection to the Internet or you like to check your mail from various locations.  With POP3 you only have one folder, the Inbox folder. When you open your mailbox, new mail is moved from the host server and saved on your computer. If you want to be able to see your old mail messages, you have to go back to the computer where you last opened your mail.  With POP3 "leave mail on server" only your messages are on the server, but with IMAP your folders are also on the server.

 Simple Mail Transport Protocol (SMTP) is the network protocol used to send across the Internet.  SMTP provides reliability as it uses TCP connection.  Current research focuses on the security issues of SMTP.