Email SMTP Simple Mail Transfer Protocol RFC 821

Email • SMTP - Simple Mail Transfer Protocol – RFC 821 • POP - Post Office Protocol – RFC 1939 • Also: – RFC 822 Standard for the Format of ARPA Internet Text Messages – RFCs 1521, 1522 Mime Netprog: Email Protocols 1

Terminology • User Agent (UA): end-user mail program • Message Transfer Agent (MTA): responsible for communicating with remote hosts and transmitting/receiving email (both a client and server). • Mail Exchanger: host that takes care of email for a domain. Netprog: Email Protocols 2

SMTP Used to exchange mail messages between mail servers (Message Transfer Agents). MTA SMTP MTA File System SMTP UA UA Netprog: Email Protocols 3

SMTP Protocol • • SMTP sender is the client SMTP receiver is the server. The communication use NVT ASCII. Alternating dialogue: – client sends command server responds with command status message. – Order of the commands is important! – Status messages include ascii encoded numeric status code (like 250, 220) and text string. Netprog: Email Protocols 4

SMTP Commands • HELO - identifies sender • MAIL FROM: - starts a mail transaction and identifies the mail originator • RCPT TO: - identifies individual recipient. There may be multiple RCPT TO: commands. • DATA - sender ready to transmit a series of lines of text, each ends with rn. A line containing only a period ‘. ’ indicates the end of the data. Netprog: Email Protocols 5

Data Format • ASCII only- must convert binary to an ASCII representation to send via email. • What if we want to send a line containing only a period? – Sender prepends a period to any line staring with a period (in the message). – Receiver strips the leading period in any line that starts with a period and has more stuff. Netprog: Email Protocols 6

Leading Period DATA 354 Start mail input; end with <CRLF> this message is a test of SMTP. . foo. . . 250 2. 6. 0 <c 2 b 1 c 1 a 0 -3951 -4 fd 6 -b 6 fb-675 c 0 ee 4 a 0 dd> Queued mail for delivery Resulting Message: this message is a test of SMTP. . foo. Netprog: Email Protocols 7

Typical Exchange wyxu@canopus % telnet hub 0. engr. sc. edu 25 Trying 129. 252. 21. 22. . . Connected to hub 0. engr. sc. edu. Escape character is '^]'. 220 HUB 0. engr. sc. edu Microsoft ESMTP MAIL Service ready at Sat, 17 Nov 2007 20: 18: 49 -0500 HELO 250 HUB 0. engr. sc. edu Hello [129. 252. 130. 106] MAIL from: <wyxu@engr. sc. edu> 250 2. 1. 0 Sender OK RCPT to: <wyxu@cse. sc. edu> 250 2. 1. 5 Recipient OK DATA 354 Start mail input; end with <CRLF> Subject: test from: sender@bla. com to: receiver@bla. com this is a test mail. 250 2. 6. 0 <c 2 b 1 c 1 a 0 -3951 -4 fd 6 -b 6 fb-675 c 0 ee 4 a 0 dd> Queued mail for delivery Netprog: Email Protocols 8

Other SMTP Commands VRFY - confirm that a name is a valid recipient. S: VRFY Smith R: 250 Fred Smith <Smith@USC-ISIF. ARPA> EXPN - expand an alias (group email address). S: R: R: R: EXPN Example-People 250 -Jon Postel <Postel@USC-ISIF. ARPA> 250 -Fred Fonebone <Fonebone@USC-ISIQ. ARPA> 250 -Sam Q. Smith <SQSmith@USC-ISIQ. ARPA> 250 -Quincy Smith <@USC-ISIF. ARPA: Q-Smith@ISI-VAXA. ARPA> 250 -<joe@foo-unix. ARPA> 250 <xyz@bar-unix. ARPA> TURN - switch roles (sender <=> receiver). Netprog: Email Protocols 9

more Commands SOML - Send Or Mail if recipient is logged in, display message on terminal, otherwise email. SAML - Send and Mail NOOP - send back a positive reply code. RSET - abort current transaction. Netprog: Email Protocols 10

Envelopes, Headers, and Body • Envelopes are used by MTAs for delivery – Specified by MAIL and RCPT commands • MAIL from: <wyxu@engr. sc. edu> • RCPT to: <wyxu@cse. sc. edu> • Headers are used by user agents – RFC 822 specifies header format – Each header field contains a name, a colon, and field value • Body is content of message from sending user to receiving user – When transferred using DATA command, headers are sent first, followed by a blank line, followed by body Netprog: Email Protocols 11

Mail Headers • Email messages contain many headers, some headers are created by the UA, some are automatically added by the MTA. • Every MTA adds (at least) a “Received: ” header. • Some of the headers are read by (parsed) intermediate MTAs, but the content is ignored and passed on transparently. Netprog: Email Protocols 12

Transmission of a Mail • User agent takes body, adds some headers, and passes to MTA • MTA adds some headers, adds envelope, and sends to another MTA Netprog: Email Protocols 13

Transmission of a mail to foo@cs. vu. nl • MTA asks the DNS for CNAME records for cs. vn. nl and the response says non exist. • MTA asks the DNS for MX records for cs. vn. nl and get one record zephyr. cs. vn. nl Portion of a possible DNS database zephyr. cs. vu. nl. top. cs. vu. nl. 86400 IN A 130. 37. 16. 112 130. 37. 16. 6 Netprog: Email Protocols 14

Transmission of a mail to foo@cs. vu. nl • MTA asks the DNS for an A record for zephyr. cs. vn. nl and get back the value of 130. 37. 16. 112. • An SMTP connection is initiated to 130. 37. 16. 112 and the mail is sent. Portion of a possible DNS database zephyr. cs. vu. nl. top. cs. vu. nl. 86400 IN A 130. 37. 16. 112 130. 37. 16. 6 Netprog: Email Protocols 15

Improvements on SMTP • Envelope changes: Extended SMTP • Header changes: Non-ASCII characters • Body changes: Multipurpose Internet Mail Extensions (MIME) Netprog: Email Protocols 16

Extended SMTP • Framework for adding extensions to SMTP • Client wishing to use new features can use EHLO, instead of HELO, to initiate session • Server lists extended commands in the 250 reply to EHLO command Netprog: Email Protocols 17

Non-ASCII Characters • Allow additional characters in sender and receiver names and in subject • Header can contain encoded words =? charset ? encoding ? encoded-text ? = – Charset can be us-ascii or iso-8859 -1 – Encoding can be quoted-printable (Q) or base-64 (B) • Using base-64, three consecutive bytes are encoded as four 6 -bit values From: =? US-ASCII? Q? Keith_Moore? = <moore@cs. utk. edu> To: =? ISO-8859 -1? Q? Keld_j=F 8 rn_Sim? = <test@gmail. com> Subject: =? ISO-8859 -1? B? SWYge. W 91 IGNhbi. By. ZWFk. IHRoa. XMge. W 8=? ==? ISO 8859 -2? B? d. SB 1 bm. R 1 cn. N 0 YW 5 k. IHRo. ZSBle. GFtc. Gxl. Lg==? = Netprog: Email Protocols 18

Multipurpose Internet Mail Extensions • Problems: – – Messages in languages with accents ( French and German) non-Latin alphabets (Russian and Hebrew) languages without alphabets (Chinese and Jamanese) not containing text at all (audio or images) • Need to add structure and define encoding rules for non-ASCII messages. Netprog: Email Protocols 19

Multipurpose Internet Mail Extensions • Allow structures in body • Five new header fields Mime-Version: Content-Type: (Type and format of the content) Content-Transfer-Encoding: (How body is wrapped for transmission) Content-ID: Content-Description: (Human-readable string telling what is in the message) • Seven content types: text, multipart, message, application, image, audio, video Mime-Version: 1. 0 Content-Type: TEXT/PLAIN; charset=US-ASCII TEXT/ENRICHED, IMAGE/JPEG, VIDEO/MPEG, MESSAGE/EXTERNALBODY, MULTIPART/ALTERNATIVE… (more examples in the book) • Five encoding formats: 7 bit, quoted-printable, base 64, 8 bit, binary Netprog: Email Protocols 20

Multipart/mixed From: Nathaniel Borenstein <nsb@bellcore. com> To: Ned Freed <ned@innosoft. com> Subject: Sample message MIME-Version: 1. 0 Content-type: multipart/mixed; boundary="simple boundary" This is the preamble. It is to be ignored, though it is a handy place for mail composers to include an explanatory note to non-MIME compliant readers. --simple boundary This is implicitly typed plain ASCII text. It does NOT end with a linebreak. --simple boundary Content-type: text/plain; charset=us-ascii This is explicitly typed plain ASCII text. It DOES end with a linebreak. --simple boundary-This is the epilogue. It is also to be ignored. Netprog: Email Protocols 21

Multipart/alternative From: Nathaniel Borenstein <nsb@bellcore. com> To: Ned Freed <ned@innosoft. com> Subject: Formatted text mail MIME-Version: 1. 0 Content-Type: multipart/alternative; boundary=boundary 42 --boundary 42 Content-Type: text/plain; charset=us-ascii. . . plain text version of message goes here. . --boundary 42 Content-Type: text/richtext. . <bold>richtext</bold> version of same message goes here. . . --boundary 42 Content-Type: text/x-whatever. . fanciest formatted version of same message goes here. . . --boundary 42 -- Netprog: Email Protocols 22

POP – Post Office Protocol • Used to transfer mail from a mail server to a UA. Mail Server POP UA File System Netprog: Email Protocols 23

POP (version 3) • Similar to SMTP command/reply lockstep protocol. • Used to retrieve mail for a single user – requires authentication • Commands and replies are ASCII lines. – Replies start with “+OK” or “-ERR”. – Replies may contain multiple lines. Netprog: Email Protocols 24

POP-3 Commands • USER - specify username • PASS - specify password • STAT - get mailbox status – number of messages in the mailbox. • LIST - get a list of messages and sizes. – One per line, termination line contains ‘. ’ only. • RETR - retrieve a message Netprog: Email Protocols 25

More POP-3 Commands • DELE - mark a message for deletion from the mailbox. • NOOP - send back positive reply • RSET - reset. All deletion marks are unmarked. • QUIT - remove marked messages and close the (TCP) connection. Netprog: Email Protocols 26

Optional Commands • TOP - send header lines from messages. • APOP - alternative authentication – APOP name digest – a string identifying a mailbox and a MD 5 digest string – `digest‘: is calculated by applying the MD 5 algorithm to a string consisting of the timestamp (sent from POP server) followed by a shared secret. – Requires shared secret! – No clear text password on the network. – Does not authenticate the server!!!! Netprog: Email Protocols 27

A Pop 3 Exchange > telnet hub 0. engr. sc. edu 110 Trying 129. 252. 21. 22. . . Connected to hub 0. engr. sc. edu. Escape character is '^]'. +OK Microsoft Exchange Server 2007 POP 3 service ready … quit +OK Microsoft Exchange Server 2007 POP 3 server signing off. Netprog: Email Protocols 28

A Pop 3 Exchange S: C: S: C: S: S: S: C: S: <wait for connection on TCP port 110> <open connection> +OK POP 3 server ready <1896. 697170952@dbc. mtview. ca. us> USER mrose +OK User accepted PASS mrosepass +OK Pass accepted STAT +OK 2 320 LIST +OK 2 messages (320 octets) 1 120 2 200. RETR 1 +OK 120 octets <the POP 3 server sends message 1>. DELE 1 +OK message 1 deleted RETR 2 +OK 200 octets <the POP 3 server sends message 2>. DELE 2 +OK message 2 deleted QUIT +OK dewey POP 3 server signing off ( maildrop empty) Netprog: Email Protocols 29

APOP S: C: S: S: S: C: +OK POP 3 server ready 1896. 697170952@dbc. mtview. ca. us APOP mrose c 4 c 9334 bac 560 ecc 979 e 58001 b 3 e 22 fb +OK maildrop has 1 message (369 octets) LIST +OK 1 messages (320 octets) 1 320. RETR 1 +OK 320 octets <the POP 3 server sends message 1>. DELE 1 +OK message 1 deleted QUIT Netprog: Email Protocols 30