Why does it do that?

"All the e-mail messages that you receive go through an e-mail server for your e-mail account. Depending on the type of e-mail account that you use, your e-mail messages are processed in one or more of the following ways.

1. Messages remain on the server. Outlook connects to the server, and you can read or delete your messages.
   
2. Messages are downloaded to a local data file called a Personal Folders file (.pst). Outlook connects to the local .pst file so that you can read and delete your messages.
   
3. Copies of messages are downloaded and synchronized with messages on the e-mail server. Microsoft Exchange Server accounts provide this functionality by creating a local Offline Folder file (.ost). This file is used for working offline when you can't connect or don't want to connect to the e-mail server. This file is also used by the Cached Exchange Mode feature in Outlook 2003.

E-mail account types differ in how e-mail messages are saved and synchronized with the e-mail server. For example, by default POP3 e-mail accounts delete e-mail messages from the e-mail server when downloaded into Outlook. However, you can customize how e-mail messages are retrieved and saved."

POP3 e-mail accounts

When you retrieve e-mail messages from a POP3 ( Post Office Protocol. A common protocol that is used to retrieve e-mail messages from an Internet e-mail server.) e-mail account, the messages are deleted from the e-mail server by default after they are downloaded to your computer. For many people, this works great.

However, if you want to check your e-mail from multiple computers, you must configure Outlook to not delete the messages on your e-mail server. This scenario is most common for people who want to check their home Internet service provider (ISP) e-mail account from work and download the messages for permanent storage on their home computer.

When you leave messages on your e-mail server, you can choose from several options to delete your messages. To make your choice, you need to consider several factors about your e-mail usage, such as how long you want the messages to be accessible from multiple computers and the storage limits imposed by your e-mail server administrator. If you exceed your storage limit, you might be unable to receive new messages or might be charged additional fees. Typically, it is best to have one computer set to the default setting that doesn't leave e-mail messages on the e-mail server. When you use the computer to retrieve your e-mail messages, they are downloaded and deleted from the e-mail server. If you choose not to do this, Outlook allows you to select a time duration for leaving e-mail messages on the server.

Also:
Wikipedia.com:
POP3

IMAP e-mail accounts

When you access an IMAP (Internet Message Access Protocol) e-mail account, your messages are copied locally. However, the messages also remain on the e-mail server until you mark them for deletion and use the Purge Deleted Items command.

Also:

HTTP e-mail accounts

When you access an HTTP e-mail account, such as MSN Hotmail, your messages are saved on the e-mail server until you delete them.

Also:
Free email providers

Exchange Server e-mail accounts

When you access an Exchange Server e-mail account, your messages are saved on the e-mail server until you delete them.

Also:
Slipstick.com:
Exchange Server
MS Exchange.org

Labels: General, Tips, Troubleshoot