Electronic mail

One of the most popular Internet services is electronic mail (e-mail).

At the beginning of the Internet era, the messages sent by electronic mail were short and consisted of text only.

Today, electronic mail is much more complex. It allows a message to include text, audio, and video.

It also allows one message to be sent to one or more recipients.

1. Electronic mail
Dr R JegadeesanProf-CSE
JyothishmathiInstitute of Technology and Science,
Karimnagar

2. •OneofthemostpopularInternetservicesis
electronicmail(e-mail).
•AtthebeginningoftheInternetera,the
messagessentbyelectronicmailwereshortand
consistedoftextonly.
•Today,electronicmailismuchmorecomplex.It
allowsamessagetoincludetext,audio,and
video.
•Italsoallowsonemessagetobesenttooneor
morerecipients.

3. Architecture
•Toexplainthearchitectureofe-mail,wegive
fourscenarios.
•Webeginwiththesimplestsituationandadd
complexityasweproceed.
•Thefourthscenarioisthemostcommonin
theexchangeofemail.

5. First Scenario
•Inthefirstscenario,thesender(Alice)andthe
receiver(Bob)ofthee-mailareusers(orapplication
programs)onthesamesharedsystem(mailserver);
theyaredirectlyconnectedtoasharedsystem(mail
server).
•Theadministratorhascreatedonemailboxforeach
userwherethereceivedmessagesarestored.
•Amailboxispartofalocalharddrive,aspecialfile
boxwithpermissionrestrictions.
•Onlytheownerofthemailboxhasaccesstoit.

6. •WhenAlice,auser,needstosendamessage
toBob,anotheruser,Alicerunsauseragent
(UA)programtopreparethemessageand
storeitinBob'smailbox.
•Themessagehasthesenderandrecipient
mailboxaddresses(namesoffiles).
•Bobcanretrieveandreadthecontentsofhis
mailboxathisconvenience,usingauser
agent.

7. Whenthesenderandthereceiverofan
e-mailareonsamesystem,weneedonlytwo
useragents

8. Second Scenario
•Inthesecondscenario,thesenderandthe
receiverofthee-mailareusers(orapplication
programs)ontwodifferentsystems.
•ThemessageneedstobesentovertheInternet.
•HereweneedUserAgents(UAs)andMessage
TransferAgents(MTAs),

10. •Aliceneedstouseauseragentprogramto
sendhermessagetothesystem(mailserver)
atherownsite.
•Thesystem(mailserver)athersiteusesa
queuetostoremessageswaitingtobesent.
•Bobalsoneedsauseragentprogramto
retrievemessagesstoredinthemailboxofthe
system(mailserver)athissite.

11. •Themessage,however,needstobesent
throughtheInternetfromAlice'ssitetoBob's
site.
•HeretwoMessageTransferAgents(MTA’s)
areneeded:oneclientandoneserver.
•Whenthesenderandthereceiverofane-
mailareondifferentsystems,weneedtwo
UAsandpairofMTAs(clientandserver).

12. Third Scenario
•Inthethirdscenario,Bob,asinthesecondscenario,is
directlyconnectedtohissystem.
•Alice,however,isseparatedfromhersystem.
•EitherAliceisconnectedtothesystemviaapoint-to-
pointWAN,suchasadial-upmodem,aDSL,ora
cablemodem;
•AliceisconnectedtoaLANinanorganizationthat
usesonemailserverforhandlinge-mails-allusers
needtosendtheirmessagestothismailserver

14. •Alicestillneedsauseragenttoprepareher
message.Shethenneedstosendthemessage
throughtheLANorWAN.
•Thiscanbedonethroughapairofmessage
transferagents(clientandserver).
•WheneverAlicehasamessagetosend,shecalls
theuseragentwhich,inturm,callstheMTA
client.
•TheMTAclientestablishesaconnectionwith
theMTAserveronthesystem,whichisrunning
allthetime.
•ThesystematAlice'ssitequeuesallmessages
received.

15. •ItthenusesanMTAclienttosendthe
messagestothesystematBob'ssite;the
systemreceivesthemessageandstoresitin
Bob'smailbox.
•Athisconvenience,Bobuseshisuseragent
toretrievethemessageandreadsit.
•Whenthesenderisconnectedtothemail
serverviaaLANoraWAN,weneedtwoUAs
andtwopairsofMTAs(clientandserver).

16. Fourth Scenario1 2 3 4 5 6 8 9

17. •Inthefourthscenarioandmostcommonscenario,
BobisalsoconnectedtohismailserverbyaWANor
aLAN.
•AfterthemessagehasarrivedatBob'smailserver,
Bobneedstoretrieveit.
•Here,weneedanothersetofclient/serveragents,
whichwecallMessageAccessAgents(MAAs).
•BobusesanMAAclienttoretrievehismessages.The
clientsendsarequesttotheMAAserver,whichis
runningallthetime,andrequeststhetransferofthe
messages.

18. •Therearetwoimportantpointshere.
•First,BobcannotbypasstheMAAmailserveranduse
theMTAserverdirectly.
•TouseMTAserverdirectly,Bobwouldneedtorunthe
MTAserverallthetimebecausehedoesnotknow
whenamessagewillarrive.
•ThisimpliesthatBobmustkeephiscomputeronall
thetimeifheisconnectedtohissystemthrougha
LAN.
•Ifheisconnectedthrougha-WAN,hemustkeepthe
connectionupallthetime.Neitherofthesesituations
isfeasibletoday.

19. •Second,notethatBobneedsanotherpairof
client/serverprograms:messageaccess
programs.
•ThisissobecauseanMTAclient/server
programisapushprogram:theclientpushes
themessagetotheserver.Bobneedsapull
program.
•Theclientneedstopullthemessagefromthe
server.

20. USER AGENT
•Thefirstcomponentofanelectronicmail
systemistheuseragent(UA).
•Itprovidesservicetotheusertomakethe
processofsendingandreceivingamessage
easier.

21. Services Provided by a User Agent

22. Composing Messages
•Auseragenthelpstheusercomposethee-
mailmessagetobesentout.
•Mostuseragentsprovideatemplateonthe
screentobefilledinbytheuser.
•Someevenhaveabuilt-ineditorthatcando
spellchecking,grammarchecking.
•Auser,ofcourse,couldalternativelyusehis
orherfavouritetexteditororwordprocessor
tocreatethemessageandimportit,orcut
andpasteit,intotheuseragenttemplate.

23. Reading Messages
•Theseconddutyoftheuseragentistoreadthe
incomingmessages.
•Whenauserinvokesauseragent,itfirstchecks
themailintheincomingmailbox.
•Mostuseragentsshowaone-linesummaryof
eachreceivedmail.
•Eache-mailcontainsthefollowingfields.
•Aflagfieldthatshowsthestatusofthemail
suchasnew,alreadyreadbutnotrepliedto,or
readandrepliedto.
•sizeofthemessagefield
•senderfield.

24. Replying to Messages
•Afterreadingamessage,ausercanusetheuseragentto
replytoamessage.
•Auseragentusuallyallowstheusertoreplytothe
originalsender.
ForwardingMessages
▪Replying is defined as sending a message to the sender or
recipients of the copy.
▪Forwarding is defined as sending the message to a third
party.
▪A user agent allows the receiver to forward the message,
with or without extra comments, to a third party.

25. Handling Mailboxes
•Auseragentnormallycreatestwomailboxes:
aninboxandanoutbox.
•Eachboxisafilewithaspecialformatthat
canbehandledbytheuseragent.
•Theinboxkeepsallthereceivede-mailsuntil
theyaredeletedbytheuser.
•Theoutboxkeepsallthesente-mailsuntilthe
userdeletesthem.

26. MIME

27. MIME(Multipurpose Internet Mail Extensions)
•MultipurposeInternetMailExtensions
(MIME)isasupplementaryprotocolthat
allowsnon-ASCIIdatatobesentthroughe-
mail.
•MIMEtransformsnon-ASCIIdataatthe
sendersitetoASCIIdataanddeliversthemto
theclientMTAtobesentthroughtheInternet.
•Themessageatthereceivingsideis
transformedbacktotheoriginaldata.

29. •MIMEdefines five headers that can be added
to the original e-mail header section to define
the transformation parameters:
1. MIME-Version
2. Content-Type
3. Content-Transfer-Encoding
4. Content-Id
5. Content-Description

31. •MIME-VersionThisheaderdefinesthe
versionofMIMEused.Thecurrentversionis
1.1.
•Content-TypeThisheaderdefinesthetype
ofdatausedinthebodyofthemessage.The
contenttypeandthecontentsubtypeare
separatedbyaslash.
•MIMEallowssevendifferenttypesofdata

33. •Content-Transfer-EncodingThisheader
definesthemethodusedtoencodethe
messagesintobinary,7bitASCII
•Content-IdItdefinesauniqueidentifier.i.e.
Thisheaderuniquelyidentifiesthewhole
messageinamultiple-messageenvironment.
•Content-DescriptionThisheaderdefines
whetherthebodyisimage,audio,orvideo

34. Message Transfer Agent: SMTP

35. Message Transfer Agent: SMTP
•Theactualmailtransferisdonethrough
messagetransferagents(MTA)
•Tosendmail,asystemmusthavetheclient
MTA,andtoreceivemail,asystemmusthave
aserverMTA.
•TheformalprotocolthatusedbytheMTA
clientandserverintheInternetiscalledthe
SimpleMailTransferProtocol(SMTP)

36. •FigureshowstherangeoftheSMTPprotocol
infourthscenario.SMTPisusedtwotimes,
betweenthesenderandthesender'smail
serverandbetweenthetwomailservers.

37. •SMTPsimplydefineshowcommandsand
responsesmustbesentbackandforth.
Commands and Responses
•SMTPusescommandsandresponsesto
transfermessagesbetweenanMTAclientand
anMTAserver

38. Commands
•Commandsaresentfromtheclienttothe
server.Theformatofacommandisshownin
figure
•Itconsistsofakeywordfollowedbyzeroor
morearguments.
•SMTPdefines14commands.

39. •Thefirstfivearemandatory;everyimplementationmust
supportthesefivecommands.
•Thenextthreeareoftenusedandhighlyrecommended.
•Thelastsixarerarelyused.

40. •HELO:Sentbyaclienttoidentifyitself
•MAILFROMIdentifiesthesenderofthe
message;usedintheformMAILFROM:
•RCPTTOIdentifiesthemessagerecipients;
usedintheformRCPTTO:
•DATASentbyaclienttoinitiatethetransfer
ofmessagecontent.
•QUITTerminatesthesession.
•RSETResettheentiremessagetransaction
andthebuffer.

41. •NOOP:Askthereceivertosendavalidreply
•VRFYVerifiestherecipientsthatamailboxis
availableformessagedelivery
•TURN:Allowstheclientandservertoswitch
rolesandsendmailinthereversedirection
withouthavingtoestablishanewconnection.
•EXPN(Expand)Askthereceivertoconfirm
thatamailinglisthasbeenidentified.
•HELPReturnsalistofcommandsthatare
supportedbytheSMTPservice.
•SENDFrom(Send)Delivere-mailtooneor
moreworkstations

42. •SOML(Sendormail)Delivere-mailtoone
ormoreworkstationsorrecipientsiftheuser
isnotactive.
•SAML(Sendandmail)Delivere-mailtoone
ormoreworkstationsandrecipientsifthe
userisnotactive.

43. Responses
•Responsesaresentfromtheservertothe
client.
•Aresponseisathreedigitcodethatmay
befollowedbyadditionaltextual
information.

45. Message Access Agent(MAA):
POP3 and IMAP4

46. MessageAccessAgent(MAA):POPandIMAP
•Thefirstandthesecondstagesofmaildeliveryuse
SMTP.
•However,SMTPisnotinvolvedinthethirdstage
becauseSMTPisapushprotocol;itpushesthe
messagefromtheclienttotheserver.
•Inotherwords,thedirectionofthebulk:data
(messages)isfromtheclienttotheserver.
•Ontheotherhand,thethirdstageneedsapull
protocol;theclientmustpullmessagesfromthe
server.
•Thedirectionofthebulkdataisfromtheservertothe
client.Thethirdstageusesamessageaccessagent.

47. •Currentlytwomessageaccessagentprotocols
areavailable:PostOfficeProtocol,version3
(POP3)andInternetMailAccessProtocol,
version4(IMAP4)

48. POP3
•PostOfficeProtocol,version3(POP3)is
simpleandlimitedinfunctionality.
•TheclientPOP3softwareisinstalledonthe
recipientcomputer.
•TheserverPOP3softwareisinstalledonthe
mailserver.
•Mailaccessstartswhentheuserclientneeds
todownloade-mailfromthemailboxonthe
mailserver.

49. •The client opens a connection to the server on
TCP port 110.
•Itthensendsitsusernameandpasswordto
accessthemailbox.Theusercanthenlist
andretrievethemailmessages,onebyone.
asshowninfigure.
•POP3hastwomodes:thedeletemodeand
thekeepmode.
•Inthedeletemode,themailisdeletedfrom
themailboxaftereachretrieval.
•Inthekeepmode,themailremainsinthe
mailboxafterretrieval.

51. Internet Mail Access Protocol, version 4
•IMAP4issimilartoPOP3,butithasmore
features;IMAP4ismorepowerfulandmore
complex.
•IMAP4providesthefollowingextrafunctions
➢Ausercancheckthee-mailheaderbefore
downloading.
➢Ausercansearchthecontentsofthee-mail
foraspecificstringofcharactersbefore
downloading.
➢Ausercancreate,delete,orrenamemailboxes
onthemailserver

52. •Ausercanpartiallydownloade-mail.Thisis
especiallyusefulifbandwidthislimitedand
thee-mailcontainsmultimediawithhigh
bandwidthrequirements.
•Ausercancreateahierarchyofmailboxesin
afolderfore-mailstorage.

53. Thank you