networking curriculum
support CD for senior
secondary studies

Topics
. Home
. Networking History
. Why network?
. OSI model
. Connecting up
. Protocols
. Technology
. Network types
. Topologies
. Issues
. Design factors
. Economic effects
. www links
. Site map

networking > protocols > smtp/pop3

SMTP/POP3

This is the TCP service that email clients such as Outlook, Outlook Express, Netscape Navigator, Opera and any other application use to gather, display, create and send emails. The email system has been widely acclaimed as the killer application on the Internet. To some extent the quick rise in popularity of the Short Message System (SMS) on telephones reinforced the opinion that simple text communication between people was extremely important and useful to a large number of users. The definition of the protocol has under-gone very few revisions since its release very early in the length of time the Internet has been in existence. The simplicity of the protocol has allowed all manner of devices from hand held organisers through to main frame computers to share messages.

To extend and reinforce the power of Telnet, if a POP3 server was opened using PuTTY as a Telnet session, it would be possible to read all a person's email using it. However this would be rather tedious and prone to error unless the user was extremely careful.

Variants on the original design have appeared. POP3 is one of the most popular protocols for retrieving email and allows mail to be downloaded from a server to a client machine. The permissions to download client email from one POP3 server can be different from the SMTP server for uploading client email. The permissions for uploading are usually linked to the username and password verified at logon time. This means only people logged on to a given ISP can use the ISP's SMTP server. In the early days of the Internet, permissions on SMTP servers were rarely set and often neglected. However people who were not really authorised abused this system by using computer programs with automated email generating systems, looking up huge databases of email addresses to bombard unsuspecting clients with unsolicited emails. This practice was called spamming as it involved much low quality communications and has been a scourge of the Internet ever since. Restricting who can logon onto the SMTP server allows ISP's to track spammers and remove them if required. However there are many ISPs who collaborate with spammers and inject huge amounts (some people have estimated it to be as high as 30% of all email traffic across the world) of email into the Internet every hour. The cost to the spammer and the ISP is small and only a few gullible people are required to respond with every mail-out to cover their costs and make money.

All sorts of bait is used to cause people to reveal their email addresses to spammers. Often the bait is only there to get the email address; often the real money is made by selling these addresses on to other techno-gullibles who want to do their own spamming with the gathered emails. So be wary of sites that request email addresses for any reason. Obvious sites to avoid are sites like pornography sites that offer free membership or other "freebie sites" (all that is required is that the password has to be emailed, hence verifying the email as a valid address) for example, but even less suspicious sites can also be used to harvest this now precious commodity. Large companies with proper privacy practices are usually very respectful of such data, however beware the registration forms that have the line at the bottom which reads like this "I wish to be notified of other special offers" and the check box is already ticked, indicating "Yes". These have become less frequent as anyone who is associated with spamming practices are seen to be rather tacky, especially ones that appear to "trick" people into accepting it.

The increase in spam over the last few years has encouraged a number of successful applications such as MailWasher, which use the POP3 protocol to download all the information from the server except the body of the message or any attachments. This allows users to scan the emails for clues as to whether the whole email is worth downloading. If the email is not downloaded the option is there to "bounce" the email, that is to make it appear the server cannot find a person with the target address. This will give the spammer the impression that the email address does not exist and that the spammer should not try that address again. To some extent this works. The most drastic but also the most effective is to simply move on to another address. Do not be tempted to enter into any return emails to have your name taken off the address list as most of these people are unscrupulous and will simple use the innocently returned email as further indication of a valid email address, and will probably not only use it again and again themselves but also sell it on to others to use. Do not open emails that have web page style content as the graphics contained in these emails are requested from URL's by the users email client and the server of these URL's can be programmed to analyse the requests for the images to work out the recipient exists and is reading the emails. Hence you get more and more of them!

The fact that POP3 can be used to download emails from different servers can be extremely handy as it can mean only one logon is required to the Internet, and once logged on any POP3 account can be accessed with the correct user name and password. Email client software such as Outlook can access these accounts automatically once set up, simply cycling through logins and downloading them all in turn. While each POP3 down load account can vary, only the ISP you have logged on to can usually provide you with a valid SMTP server to send email out from your machine.

Finally email has a simple mode of file transfer where strings of characters terminated with a line feed or carriage return character indicated a 'line'. Email allowed exchange of these lines of characters over the TCP. However because the formats for what constituted a properly terminated line differed across operating systems, the sequence of characters often involved non-printable characters and in non- English language style combinations depending on what was being exchanged. When simple text was being transmitted, that is the data followed basic rules of language (printable characters with end of lines terminated with a line feed for example) the email system worked well. However when the contents of the file were an image or an executable, for example, the sequences of characters rarely conform to this pattern and also include many non-printable characters. To deal with these files they are encoded into pairs of ASCII characters in such a way that the characters were always "printable", terminated with proper terminators and the lines were never too long (some relay servers still terminate long ASCII lines in these protocols).

Examples of this style of encoding is UUEncode, BINHEX(5) and MIME. They are not to be confused with compression systems that reduce the size of the file by encoding redundancy into structured metabit patterns. Rather, these systems translate the full range of binary bytes found in files like executables and image files into a predefined encoding that uses only printable characters. This often increases the size of the attachment but allows the file to be "attached to" (actually inserted within) the email). Using matching operating systems makes transfers easy as the encoding techniques will most likely match. However swapping attachments between Microsoft Windows and Apple operating systems for example, can sometimes be quite difficult.

worksheets
. history
. why network
. osi model
. connecting
. protocols
. technology
. topologies (tla)
. issues
. network types
. design
. economics
. economics (cs)
   - solutions -

quizzes
. why network
. osi model
. connecting
. protocols
. technology
. topology
. issues
. network types
. design
. economics

| Home | History | Why Network? | OSI Model | Connecting up | Protocols | Technology | Networks Types | Topologies | Issues | Design Factors | Economic effects |
:: copyright | credits | sitemap ::

© 2003 Victorian Information Technology Teachers Association
http://www.vitta.org.au/