As its name suggests, Internet Mail Access Protocol, better known as IMAP, is designed to manage messages stored on the Internet, which means you can access your mail from any computer, anywhere. If you understand how IMAP works and take reasonable care to avoid unnecessary downloads it is a very powerful and flexible mail manager that can deliver significant performance benefits particularly on slow (e.g. dial) connections.
An email message consists of a header and one or more body parts and/or attachments. The header is always small but body parts and attachments can vary from very small to very large. When an IMAP folder is selected, message headers are displayed. For each message you can see who it is from, how big it is, when it was sent, etc.. If you have set OE up correctly no message is actually downloaded until you specifically ask for it to be downloaded. This lets you decide what to do with each message according to its characteristics (size, subject, sender) which you know from the information in the header.
Outlook Express is a good IMAP mail client if you follow the configuration guidelines described in the relevant sections of this document. It has the advantage of being free (included with every copy of Windows), relatively easy to use, and it supports both POP and IMAP mail accounts. Many other free and fee mail clients are available and which one(s) you eventually settle on is really a matter of personal preference. The scope of this document is limited to OE but once you are comfortable with IMAP you will be well placed to try other clients and compare features and benefits.