Post Office Protocol (POP)

With a POP account, incoming messages are stored on the ISP’s mail server and are downloaded to your computer when you access your account. The messages remain on the server until you delete them. Your mailboxes and messages are available only from one computer.

Internet Message Access Protocol (IMAP)

With an IMAP account, incoming messages are stored on the ISP’s mail server until you delete them. When you first connect to the mail server, all your mailboxes are automatically synchronized so that the messages on your computer are the same as those on the server.

IMAP accounts are especially useful if you need to read and manage messages from more than one computer. Your mailboxes are visible on any computer you use to access your account, and you can work with messages as if they were stored on your computer. You can choose to download messages or attachments to your computer, so you can work with them when you're not connected to the Internet. Some ISPs restrict the number of messages you can store on the IMAP server and the size of messages and attachments.