A mail::ACCOUNT object represents a "mail account". An account is a collection of folders (mail::folder objects). The folders are arranged in a hierarchy, top to bottom. A folder must be "opened", before messages in the folder can be read. One folder, per account, can be opened at a time. Opening another folder closes the previous folder. mail::ACCOUNT objects contain methods for enumerating and navigating the mail::folder objects, and for accessing messages in the currently-open folder.