Another important concept is a means for grouping messages together. All email software provides some means for doing this, and IMAIL is no exception. IMAIL provides objects called folders. A folder is just an object that holds an arbitrary number of email messages. Messages can be added to a folder, deleted from a folder, and moved or copied from one folder to another.
In IMAIL, the concept of the folder is used to embrace different grouping mechanisms. This is because IMAIL provides a uniform means for accessing different kinds of email systems. In particular, IMAIL supports access to Emacs Rmail files (also known as BABYL files, for historical reasons), to unix mailbox files (sometimes called mbox files), and to IMAP mailboxes. Each of these grouping mechanisms, although implemented very differently, is viewed as a folder by IMAIL. With some exceptions, each of these different types of folder are treated exactly the same by IMAIL. Finally, because IMAIL is extensible, other types of folders may be supported in the future.