Electronic Mail and SMTP 1 Electronic Mail System

Electronic Mail and SMTP 1

Electronic Mail System components: • user agents • mail servers • simple mail transfer protocol: SMTP – TCP port 25 outgoing message queue user mailbox user agent mail server SMTP User Agent SMTP • a. k. a. “mail reader” • composing, editing, reading mail messages mail server • e. g. , Eudora, Outlook, elm, Netscape Messenger • outgoing, incoming messages user stored on server SMTP user agent mail server user agent 2

Electronic Mail: Issues ISSUES 1. How does UA talk to mail server to send a mail 2. How does a mail server talk to another mail server to send a mail 3. How does UA talk to mail server to retrieve a mail 1 and 2 – SMTP 3 – PO 3, IMAP SMTP mail server user agent POP 3 IMAP SMTP mail server user agent SMTP user agent mail server user agent 3

Electronic Mail: SMTP [RFC 2821] • uses TCP to reliably transfer email message from client to server, port 25 • direct transfer: sending server to receiving server • three phases of transfer – handshaking (greeting) – transfer of messages – closure • command/response interaction – commands: ASCII text – response: status code and phrase • messages must be in 7 -bit ASCII 4

Scenario: Alice sends message to Bob 4) SMTP client sends Alice’s message over the TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his user agent to read message 1) Alice uses UA to compose message and “to” bob@someschool. edu 2) Alice’s UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bob’s mail server 1 user agent 2 mail server 3 mail server 4 5 6 user agent 5

Sample SMTP interaction S: C: S: C: C: C: S: 220 hamburger. edu HELO crepes. fr 250 Hello crepes. fr, pleased to meet you MAIL FROM: <alice@crepes. fr> 250 alice@crepes. fr. . . Sender ok RCPT TO: <bob@hamburger. edu> 250 bob@hamburger. edu. . . Recipient ok DATA 354 Enter mail, end with ". " on a line by itself Do you like ketchup? How about pickles? . 250 Message accepted for delivery QUIT 221 hamburger. edu closing connection 6

SMTP: final words • A “PUSH” protocol • In-band protocol unlike FTP • A single message body even if the message consists of several parts • Requires message (header & body) to be in 7 -bit ASCII • Server uses CRLF to determine end of message 7

Mail message format RFC 822: standard for text message format: • header lines, e. g. , – To: – From: – Subject: different from SMTP commands! • body header blank line body – the “message”, ASCII characters only. – OK for text messages • What about sending a jpeg image, which is not ASCII – Encode it to 7 -bit ASCII – How does the receiver know that the message is a jpeg image? • Need additional headers called the Multipurpose Internet Mail Extension 8 (MIME) headers

MIME Headers • MIME: multimedia mail extension, RFC 2045, 2056 • additional lines in msg header declare message content type MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: alice@crepes. fr To: bob@hamburger. edu Subject: Picture of yummy crepe. MIME-Version: 1. 0 Content-Transfer-Encoding: base 64 Content-Type: image/jpeg base 64 encoded data. . . . . base 64 encoded data 9

MIME types Content-Type: type/subtype; parameters Text Video Image Application • example subtypes: plain, html • example subtypes: jpeg, gif Audio • example subtypes: mpeg, quicktime • other data that must be processed by reader before “viewable” • example subtypes: msword, octet-stream • exampe subtypes: basic (8 -bit mu-law encoded), 32 kadpcm (32 kbps coding) 10

Multipart Type From: alice@crepes. fr To: bob@hamburger. edu Subject: Picture of yummy crepe. MIME-Version: 1. 0 Content-Type: multipart/mixed; boundary=Start. Of. Next. Part --Start. Of. Next. Part Dear Bob, Please find a picture of a crepe. --Start. Of. Next. Part Content-Transfer-Encoding: base 64 Content-Type: image/jpeg base 64 encoded data. . . . . base 64 encoded data --Start. Of. Next. Part Do you want the reciple? 11

Mail access protocols user agent SMTP sender’s mail server • • access protocol user agent receiver’s mail server SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server – POP: Post Office Protocol [RFC 1939] – TCP port 110 • authorization (agent <-->server) and download – IMAP: Internet Mail Access Protocol [RFC 1730] • more features (more complex) • manipulation of stored msgs on server – HTTP: Hotmail , Yahoo! Mail, etc. 12

POP 3 protocol authorization phase • client commands: – user: declare username – pass: password • server responses – +OK – -ERR transaction phase, client: • list: list message numbers • retr: retrieve message by number • dele: delete • quit S: C: S: +OK POP 3 server ready user bob +OK pass hungry +OK user successfully logged C: S: S: S: C: C: S: list 1 498 2 912. retr 1 <message 1 contents>. dele 1 retr 2 <message 1 contents>. dele 2 quit +OK POP 3 server signing off 13 on

POP 3 (more) and IMAP More about POP 3 • Previous example uses “download and delete” mode. • Bob cannot re-read email if he changes client • “Download-and-keep”: copies of messages on different clients • POP 3 is stateless across sessions IMAP • Keep all messages in one place: the server • Allows user to organize messages in folders • IMAP keeps user state across sessions: – names of folders and mappings between message IDs and folder name 14