Some Internet service providers (ISPs) offer mail accounts that use Internet Message Access Protocol (IMAP).

With an IMAP account, your incoming email remains on your ISP's mail server until you delete it. You can read and manage your email from different locations. Your In, Out, Sent, and Trash mailboxes, plus any mailboxes you create, are visible to you no matter what computer you use to access your mail account, and you can work with your messages as if they were stored on your computer. For this reason, you may want to use an IMAP account if you usually read your email at more than one computer.

You can control whether messages are stored on your local hard disk. For example, you can choose to download messages only when you read them, or download attachments only when you open a message. This can allow for better performance if you use a telephone modem to connect to the Internet.

You can choose to keep a copy of your mail on your computer so you can work with it when you're not connected to the Internet. This option, sometimes referred to as "caching," is useful if you sometimes use your computer when an Internet connection is not available. You can set up caching by opening the Accounts pane of Mail Preferences, selecting an account, and clicking the Advanced button. Choose which messages you want to cache using the “Keep copies of messages for offline viewing” pop-up menu.

Not all ISPs allow you to use IMAP accounts. Also, there are usually restrictions on how many messages you can keep on the IMAP server and how large each message can be. Contact your Internet service or email provider for details about what is available to you.