POP3 and default IMAP access are optimized for downloading email to your email application at your home, office or mobile computer. Common generic mail system is not optimized for working with large count of emails on our servers.
Extended IMAP mailbox is special mailbox mode, enabled free of charge. It allows efficient work with your emails on our server and much larger email store.
UNIX administrator may enjoy using this feature as personal UNIX-style single information point.
Extended box is made from regular mailbox. Convert needed mailboxes to extended mode using "Extended IMAP mailbox" page of your Control Panel.