Email software such as Outlook Express often uses the POP3 format by default when checking the messages stored on your email server. POP3 works by downloading any new emails to your computer and then deleting them from the server.
This might be fine if you're only checking your email from one computer, but if you're jumping between multiple computers and other gadgets then it can leave your inbox scattered across different devices.
One workaround is to only access your email via a web browser, but webmail interfaces are often rather limited compared to a desktop mail client. Another alternative is to configure your mail client such as Outlook, Thunderbird or Entourage to read your email using IMAP rather than POP3. IMAP downloads emails to your device but doesn't delete them from the server.
But does however mark emails as read and it also stores your sent messages on the server. This means you can read your email from any of your devices and have your full inbox at your disposal.
IMAP is also useful for when multiple staff members need access to the one email account.