IMAP implements server-based folders and mail storage, providing the ability to synchronize server-based folders with copies stored on a client system. When new mail is delivered to an account, it's placed in the Inbox folder. Whenever a message is deleted from the Inbox folder or moved to another folder, that message is deleted from the server Inbox folder. POP3 clients such as Outlook Express and Outlook's Internet Mail account type see only the Inbox folder; they don't know of the existence of other IMAP folders the account owns. By default, most POP3 clients delete messages from the server after downloading them and place the messages in local storage--an Outlook Personal Folder file (PST), for example. So, unless your POP3 client is set to leave messages on the server, downloaded messages won't be available to IMAP or Web clients. Similarly, if you have rules in your IMAP client that move messages from the Inbox to other folders, your POP3 client won't see the messages. In most cases, it's best--and easiest--to use only your IMAP and Web clients against IMAP account structures.
