From Heinz Tschabitscher,
Your Guide to Email.
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If you use email, I'm sure you have heard somebody heard talking about Hotmail's lack of "POP access" or were told to configure the "POP server" in your email client.

So what is this POP that is obviously so important if you want to get your emails?

The Purpose of POP, the Post Office Protocol

If somebody sends you an email it usually cannot be delivered directly to your computer. The message has to be stored somewhere, though. It has to be stored in a place where you can pick it up easily. Your ISP (Internet Service Provider) is online 24 hours on 7 days of the week and will do that job. It receives the message for you and keeps it until you download it.

Let's suppose your email address is look@me.com. As your ISP's mail server receives email from the internet it will look at each message and if it finds one addressed to look@me.com that message will be filed to a folder reserved for your mail.

This folder is where the message is kept until either you retrieve it or one of your ISP's administrators finds your account has been filled with spam and decides to delete all the mail in it (no, this won't happen but it can very well happen that you go past your limit of space available for incoming messages and cannot receive any more).

Now, POP, the Post Office Protocol (as defined in RFC 1939) is what allows you to retrieve mail from your ISP. This is also about all the Post Office Protocol is good for.

What the Post Office Protocol Allows You to Do

Like it seems everything on the internet, mail retrieval is a client-server application. The Post Office Protocol defines how your email client should talk to the POP server. The POP is a very simple protocol. This makes it easy to implement, has earned the Post Office Protocol widespread adoption and makes it very robust, but it also means the Post Office Protocol provides only basic functionality.

Things that can be done via the POP include:

• Retrieve mail from an ISP and delete it on the server.
• Retrieve mail from an ISP but not delete it on the server.
• Ask whether new mail has arrived but not retrieve it.
• Peek at a few lines of a message to see whether it is worth retrieving.

Of these, the second probably sounds the most dangerous. Deleting something is always frightening. Remember, though, that you retrieve your mail before you delete it and thus have a copy.

If you leave all your mail on the server, it will pile up there and eventually lead to a full mailbox. When your mailbox is full, nobody will be able to send you any email before you haven't cleaned up.