Information Technology Reference
In-Depth Information
received on the addressee's mail service provider server. The addressee then
views from, or retrieves the message to his/her own computer using a mail
client(whichmaybeawebbrowser).Usually,themessageremainsonthemail
serveruntiltheuserdeletesit(orperhapstheserviceproviderdeletesitaftera
certain amount of time, due to policy). Messages may be addressed to a single
recipient or multiple recipients. When using email to a group of recipients
which is relatively unchanging, or if managing the list of recipients becomes
complicated, it is useful to use an email listserver.
These days, email is almost entirely based on internet standards, primarily
SMTP (Simple Mail Transport Protocol), so mail from one application can
almost always be read with a different application or sent from one type of
server to another type. In some cases, a protocol bridge is used to translate
betweenSMTPandaproprietaryprotocol,forexampleLotus/MailorSoftArc
FirstClass. The SMTP standard assumes text-based messages, and in particu-
larassumestheASCII(AmericanStandardCodeforInformationInterchange)
character codes (essentially the US English alphabet plus digits and a limited
selection of punctuation marks). It is possible, however, to transmit other
alphabets as well as images and other media by encoding the data as MIME
(Multi-purpose Internet Mail Extensions). Most mail applications will handle
the encoding when necessary; however there remain a significant number of
users with purely text-based email readers, for which these MIME-encoded
messagesmaycausedifficultiesbybeingrenderedasanunrecognizableencod-
ing rather than the original text.
The SMTP standard details both how the mail servers communicate with
each other, and what format the actual mail messages take. Every message
begins with several lines that are called the header and contain message meta-
data.Oneblanklinefollowstheheader,andtherestofthemessagecontainsthe
user content, perhaps encoded. The header lines have a parameter name, fol-
lowedbyacolon,andfollowedbythevalueoftheparameter.Commonlyknown
parametersareFrom,To,DateandSubject,withfairlyobviousmeanings.These
areusuallydisplayedbymailclientsinornearthemessagewindow.Otherpara-
meters are less known because they are usually not displayed by default. A few
of the important ones (for our purposes) are Return-Path, Message-Id, In-
Reply-ToandReferences.TheReturn-Pathdescribeswhowillreceiveareplyto
the current message. It is usually the same address as From, but it need not be.
There are legitimate reasons for From and Return-Path to be different, but it is
also a feature that is abused by scammers. They generate a message that seems
tocomefromafinancialinstitutionandhasaFromaddressthattheusermight
recognize, but the Return-Path may point to someone trying to harvest user
information for malicious purposes. For email exchanges the Message-Id, In-
Reply-To and References parameters are important. An example will illustrate:
three very short messages follow from an exchange with replies. All are dis-
playedinrawformattoshowthemailheaders.Thefirstistheoriginalmessage.
