There are three types of e‑mail servers. You don’t need to understand the details about these server types; you just need to know there uses since Kaponline supports both POP3 and IMAP:

• Post Office Protocol 3 (POP3) servers. Most users use this type of server, especially for personal e‑mail accounts. They hold incoming e‑mail messages until you check your e‑mail, at which point they're transferred to your computer. Messages are typically deleted from the server when you check your e‑mail.
• Internet Message Access Protocol (IMAP) servers. This service let you work with e‑mail messages without downloading them to your computer first. You can preview, delete, and organize messages directly on the e‑mail server. Copies are stored on the server until you delete them. IMAP is commonly used for business e‑mail accounts or email accounts that are shared between devises such as computers and smart phones.
• Simple Mail Transfer Protocol (SMTP) servers. This outgoing mail server handles the sending of your e‑mail messages to the Internet. An SMTP server handles only the outgoing e‑mail, and is used in conjunction with a POP3 or IMAP incoming e‑mail server.

extracts from: http://windows.microsoft.com/en-US/windows-vista/Windows-Mail-setting-up-an-account-from-start-to-finish