POP3 typically operates in the off-line mode. By contrast, online users access mailboxes that remain on the mail server but are still manipulated by clients. The mail doesn't leave the server's storage. Online mode is mainly used by stationary users, particularly across a fast LAN connection, but it also works well with low-powered computers dialing in from remote locations. Some POP3 servers offer online capabilities but not at the same level as IMAP4 does.
Disconnected mode offers the most flexibility.
The client software copies, or caches, messages and attachments selected by the user to a local disk and leaves the original copy on the mail server. The messages in cache can be manipulated and later re-synchronized with the mail server after reconnecting. At present, this feature is implemented on mail server programs, but few clients available today support disconnected mode. Of the products in our roundup, Sun's Internet Mail Client was the only one that offered this capability. An ACAP-compliant client lets you access multiple IMAP4, Revision 1 mailboxes on multiple servers.