What is POP3?



POP stands for Post Office Protocol. It is the protocol (a common language that
is used by computers to exchange information between different hardware components,
different computers over a network etc'. A computer protocol is pretty much the
same as a regular human language) used to check any non-web-based Email account.




A web-based Email account is an Email box that can only be used through a certain
website. During your stay in this website, you are exposed to ads which the company
who provides you with this mailbox profits from. Web-based Email services are
always free because instead of paying, you pay the company who handles your mailbox
by watching these ads (unless this mailbox features some extremely special services
or features that cost more money).



Hence, a non-web-based Email account is an account that is accessed using an Email
client or a regular telnet client, if you know the necessary protocols (after
you finish reading this tutorial and the Sendmail tutorial, you would know how
Email works and how to compose and receive Email without the need of any program
besides a telnet client).



Non-web-based Email accounts use two protocols - SMTP (Simple Mail Transfer Protocol)
to send Email and POP (Post Office Protocol) to receive Email. You can study the
SMTP protocol in the Sendmail tutorial, and as for POP - you'll get to learn it
right now. In case you're wondering, POP3 is just a newer version of POP.





Why would I want to learn the POP protocol?



Oh, there are a lot of reasons why you would want to do that. I'll give you a
couple of examples out of my head:



a. Have you ever been mail-bombed? If not, can you imagine how it is
to have to receive and then delete hundreds or even thousands of messages? Well,
no more! After you'll read this tutorial you would be able to delete Email right
off the mail server without having to receive it and then delete it from your
Email client (some Email clients won't let you group several mails and delete
all of them in one shot so you have to delete them one by one, which would make
your job even more difficult).



b. You would have the ability to read your Email from anywhere in the
world. Sure, many people would say \"but we can always go to www.readmail.com,
enter our Email address and our password and we'll be presented with a nice interface
which will let us read our Email and delete messages from anywhere around the
world. Why would we want to learn the POP3 protocol instead?\" Well, there's a
serious security problem with using websites such as www.readmail.com. For example:
some browsers support an option called \"form completion\". The browser records
the data you type into forms and then, when you return to a form and start filling
it out, it will fill all the rest for you.



Now, what happens if you use www.readmail.com on some computer, and then someone
else comes over and fills in your Email address? That's right! The browser will
complete the rest of the form, thus reveal your password! This was just an example
of what insecurities readmail.com might pose.



c. The POP3 protocol isn't so complicated. In fact, it's quite easy,
so hell, why not learn it just for fun, huh? :-)



Alright, let's go!



The first thing you will need is a regular telnet application. No need for anything
fancy. Unix has a native telnet client (simply type telnet hostname/IP to telnet
to that hostname or IP into port 23 or telnet hostname/IP port to choose a port
other than 23). Windows 95 and above also come with a telnet application (it's
in c:\\windows\\telnet.exe, but you can also press start, run and type telnet to
run it, telnet hostname/IP to telnet to that hostname or IP and into port 23 or
specify a port other than 23 with telnet hostname/IP port).



Next, you need to find your POP3 mail server. Usually it would be something like
mail.your-isp.net. For example: my ISP is netvision.net.il, so my POP3 server
is located at mail.netvision.net.il. If you already have an Email client that
is configured correctly for your POP3 box, go to your preferences dialog boxes
and you should be able to find your incoming mail server address there.



Okay, looks like we're pretty much ready for this. Next thing, we telnet to our
POP3 server and into port 110 (the default port for POP3). This is what I get
when I telnet into my POP3 page:



+OK POP3 [194.90.1.6] v7.60 server ready



Okay, looks good so far. Now we need to identify ourselves. We need to supply
our username and password. Your username would usually be in your Email address.
For example: my Email address is barakirs@netvision.net.il, therefore my username
is barakirs. So we type:



user barakirs (or whatever username you have)



And the server replies with:



+OK User name accepted, password please



Next thing, we need a password, right? So we do:



pass your-password



And the server replies with:



+OK Mailbox open, x messages (when x is the number of messages you have in your
inbox)



Or with:



-ERR Bad login



If we supply the wrong password. Now, we want to see what we have, so we first
type list to see the list of Emails we currently have (unless, of course, we have
0 new messages). For each Email, we would get a separate line which will contain
a message number (a number that identifies each message) and a number that would
indicate the size of each message.



Next thing, we want to read our messages. To do this, we type retr #. Retr stands
for retrieve. # should be replaces by the message's number. We would then get
a full message header (see Sendmail tutorial) and the body of the message.



After we read the message, we would probably want to delete it. In order to do
that, we type dele #. Dele stands for delete. # should be replaced by the message's
number.



Once you're finished, type quit to close your session.



Final notes ........... Yup, that's all! Oh, just one more thing - when you log
into your POP3 mailbox, it gets locked up so noone can access it while you're
inside. A \"lock file\" is created when you log in and is deleted when you log out,
and whenever someone tries to log in the server allows him access only if the
lock file does not exist (which means that there's nobody currently logged in).




See? It's easy. I hate to tell you \"I told you so\" but... :-)