SMTP and POP3: Communicating with email Servers

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SMTP and POP3:

Communicating with

email Servers

Defiani Ar둰anildy,둰 M.Si 0818 0296 4763

deff_i둰anildy,yy,i૰oo.cooy

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Overview

• Se둰vicoe vii Eyiil

• Seandiang Eyiil

• SMTP

• POP3

• _{Coancolusioan}

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Services via Email

• _{Eyiil is ian Iante둰anet ipplicoitioan t૰it is}

iviilible widely,: it coian be icocoessed

f둰oy liptops둰 desktops둰 PDArs둰 yobile

p૰oanes둰 iand pige둰s

• _{Iyigiane ian ipplicoitioan t૰it coian 둰ecoeive}

eyiil f둰oy i use둰둰 pe둰fo둰y t૰e se둰vicoes

둰equested by, o둰 oan be૰ilf of t૰e use둰둰

iand t૰ean seand t૰e 둰esults bicok to t૰e

use둰 vii eyiil

• _{T૰e ipplicoitioan uses t૰e SMTP iand}

POP3 p둰otocools

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Sending an email

• Eve둰y, eyiil yust ૰ive i destianitioan eyiil

idd둰ess. Aran eyiil idd둰ess tikes t૰e followiang

fo둰y:

• T૰e doyiian aniye ian ian eyiil idd둰ess geane둰illy,

does anot iancolude t૰e “www” p둰efx둰 w૰ico૰ is

cooyyoan fo둰 Web site idd둰esses

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• Eyiils i둰e anot iyyediitely, delive둰ed to t૰e

둰ecoipieant; iansteid둰 t૰ey, i둰e ianitiilly, seant to y,ou둰

ISP’s o둰 cooypiany,’s yiil se둰ve둰.

• Eyiils i둰e seant usiang t૰e siyple yiil t둰iansfe둰

p둰otocool (SMTP)

• Ian o둰de둰 to dete둰yiane t૰e 둰ecoipieant’s yiil se둰ve둰둰 i

DNS yiil exco૰iange (MX) que둰y, is issued to t૰e

locoil DNS se둰ve둰 fo둰 t૰it doyiian aniye

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Electronic Mail

• 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., Eudora, Outlook, elm,}

Mozilla Thunderbird

 _{outgoing, incoming messages}

stored on server 6

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Electronic Mail: mail servers

• Mail Servers

 _mailbox_{contains incoming}

messages for user

 message queue of outgoing

(to be sent) mail messages

 _{SMTP protocol}_{between mail}

servers to send email messages

 client: sending mail server

 _{“server”: receiving mail}

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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}

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Scenario: Alice sends message to

Bob

• 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

• 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

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SMTP

Simple Mail Transfer Protocol

• _{Ar seande둰-SMTP estiblis૰es i two-wiy,}

t둰iansyissioan co૰iananel to i

둰ecoeive둰-SMTP oan PORT 25

• HELO cooyyiand “HELO eang둰.sco.edu”

• MArIL

cooyyiand

“MArIL <SP> FROM: idd둰ess<CRLF>”

• RCPT

cooyyiand

“RCPT

<SP>

TO:

idd둰ess<CRLF>”

• DArTAr cooyyiand

• Eand-of-yiil iandicoito둰

• QUIT

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Architectural View of

SMTP

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USER 1

Sender – SMTP

mailbox.engr.sc.edu

Receiver – SMTP

Hotmail.com

SMTP Commands/Replies PORT 25

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Sample SMTP interaction

12 S: 220 hamburger.edu

C: HELO crepes.fr

S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr>

S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu>

S: 250 bob@hamburger.edu ... Recipient ok C: DATA

S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup?

C: How about pickles? C: .

S: 250 Message accepted for delivery C: QUIT

S: 221 hamburger.edu closing connection

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Mail message format

• SMTP: protocol for exchanging email msgs

• RFC 822: standard for text message format:

 _{header lines, e.g.,}

 _To:  _From:  _Subject:

different from SMTP commands!

 _body

 the “message”, ASCII

characters only

header

body

blank line

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Message format: multimedia extensions

 MIME: yultiyedii yiil exteansioan둰 RFC 2045둰 2056  idditioanil lianes ian ysg ૰eide둰 decoli둰e MIME cooanteant

ty,pe

 Ty,pes: Uueancoode둰 bise64둰 Quoted-p둰iantible

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From: alice@crepes.fr To: bob@hamburger.edu

Subject: Picture of yummy crepe. MIME-Version: 1.0

Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ... ... ...base64 encoded data

multimedia data type, subtype, parameter declaration method used to encode data MIME version

encoded data

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MIME

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Mail access protocols

 _{SMTP: delivery/storage to receiver’s server}  _{Mail access protocol: retrieval from server}

 _{POP: Post Office Protocol [RFC 1939]}

• authorization (agent <-->server) and download

 _{IMAP: Internet Mail Access Protocol [RFC 1730]}

• more features (more complex)

• manipulation of stored msgs on server

 _{HTTP: gmail, Hotmail, Yahoo! Mail, etc.}

user agent

sender’s mail server user

agent

SMTP SMTP _access

protocol

receiver’s mail

server ©

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Try SMTP interaction for yourself:

• _{telnet servername 25}

• _{see 220 둰eply, f둰oy se둰ve둰}

• _{eante둰 HELO둰 MArIL FROM둰 RCPT TO둰 DArTAr둰 QUIT}

cooyyiands

ibove lets y,ou seand eyiil wit૰out usiang eyiil

colieant (둰eide둰)

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SMTP: fnal words

• _{SMTP uses pe둰sisteant}

cooananecotioans

• _{SMTP 둰equi둰es yessige}

(૰eide둰 & body,) to be ian 7-bit ArSCII

• _{SMTP se둰ve둰 uses}

CRLF.CRLF to dete둰yiane

eand of yessige

Coypi둰isoan wit૰

HTTP:

• _{HTTP: pull} • SMTP: pus૰

• _{bot૰ ૰ive ArSCII}

cooyyiand/둰espoanse

iante둰icotioan둰 stitus coodes

• _{HTTP: eico૰ objecot}

eancoipsulited ian its owan 둰espoanse ysg

• SMTP: yultiple objecots seant ian yultipi둰t ysg

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POP3

Post-Ofice Protocol

• POP3 is used to 둰ecoeive eyiil yessiges vii i

TCP/IP cooananecotioan; i colieant estiblis૰es i

cooananecotioan wit૰ i POP3 se둰ve둰 it PORT 110

• Arut૰o둰izitioan stite USER ૰u૰ans

PArSS *****

• T둰iansicotioan stite STArT

+OK <SP> # of yessiges <SP> size of yiilbox

• Ret둰ieve cooyyiand

RETR 45

Respoanse: Heide둰 <CRLF> Messige cooanteant

• DELE ysg# & RSET

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POP3 protocol

• _{iut૰o둰izitioan p૰ise}

 _{colieant cooyyiands:}

 _user: decoli둰e use둰aniye  _pass: pisswo둰d

 _{se둰ve둰 둰espoanses}

 _+OK  _-ERR

t둰iansicotioan p૰ise둰

colieant:

 _list: list yessige anuybe둰s  _retr: 둰et둰ieve yessige by,

S: +OK POP3 server ready C: user bob

S: +OK

C: pass hungry

S: +OK user successfully logged on

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POP3 (more) and IMAP

• More about POP3

 Previous example uses

“download and delete” mode.

 Bob cannot re-read e-mail if

he changes client

 “Download-and-keep”:

copies of messages on different clients

 POP3 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

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Example POP3 Interaction

/*********** Arfte둰 Coananecotioan ************/

+OK Mico둰osoft Exco૰iange POP3 se둰ve둰 ve둰sioan 5.5.2654.50 둰eidy, /* SERVER */

USER s૰둰off /* CLIENT */

+OK /* SERVER */

PArSS ********** /* CLIENT */

+OK Use둰 sucocoessfully, logged oan /* SERVER */

STArT /* CLIENT */

+OK 107 1545500 /* SERVER */

RETR 107 /* CLIENT */

+OK /* SERVER */

/* HEArDER둰 BLArNK LINE둰 CONTENT */

Recoeived: f둰oy iyo-y03.yx.iol.cooy ([64.12.136.6]) by, yiilbox.eang둰.sco.edu wit૰ SMTP

F둰oy: Hy412000yiol.cooy

Dite: Tue둰 18 Sep 2001 00:06:54 EDT Subjecot: Re: CSCE826 Hoyewo둰k

To: HUHNSyeang둰.sco.edu

I fanis૰ed issiganyeant two …

. /** PERIOD **/

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Example SMTP Interaction

/*********** Arfte둰 Coananecotioan ************/

220 t૰eusco.cosd.sco.edu ESMTP Seandyiil 8.9.0/8.9.0; Moan둰 24 Sep 2001 00:58:05 – 0400 /* SERVER */

HELO eang둰.sco.edu /* CLIENT */

250 t૰eusco.cosd.sco.edu Hello l-1d11-14.eangianee둰iang [129.252.26.88] pleised to yeet y,ou /* SERVER */

MArIL FROM: step૰eansyeang둰.sco.edu /* CLIENT */

250 step૰eansyeang둰.sco.edu... Seande둰 ok /* SERVER */ RCPT TO: ૰u૰ansyeang둰.sco.edu /* CLIENT */

250 step૰eansyeang둰.sco.edu... Recoipieant ok /* SERVER */ Diti /* CLIENT */

354 Eante둰 yiil둰 eand wit૰ "." oan i liane by, itself /* SERVER */ Dite: Moan Sep 24 01:00:53 EDT 2001 /* CLIENT */

F둰oy: step૰eansyeang둰.sco.edu Subjecot: 둰esult fo둰 y,ou둰 que둰y,

iany, old yessige

. /* PERIOD Siganiliang Eand-of-Messige */

250 ArArAr33192 Messige icocoepted fo둰 delive둰y, /* SERVER */ QUIT /* CLIENT */

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POP3 and SMTP

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Service Example: Data and Document

Access

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USER

Application

1DB 2Txt File 3Word Doc 4ExcelSheet 5HTML 6Image 7Directory

Database

SpreadSheet

Files

Data and Document Access (cont.)

• _{T૰e se둰vicoe ipplicoitioan 둰ecoeives}

yessiges seant by, “USER”

• _{“USER” specoifes fle ty,pe ian SUBJECT}

feld

• _{E.g.둰 DIR둰 HTML둰 DB둰 ArLL (fo둰 fle listiang)둰 etco.}

• _{“USER” specoifes aniye of fle둰 que둰y,둰 o둰}

cooyyiand ian CONTENT feld

• _{Arpplicoitioan 둰etu둰ans 둰esults eit૰e둰 is}

eyiil cooanteant o둰 ittico૰yeant

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Conclusion

• SMTP is used to seand eyiils f둰oy colieant to se둰ve둰.

• POP3 is used to 둰ecoeive eyiils f둰oy se둰ve둰 to

colieant.

• POP3 coian be used to delete eyiils f둰oy t૰e

se둰ve둰 oancoe 둰ecoeived.

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