.IMAPMBOX File Extension

Folder created by Apple Mail, the default email client for Mac OS X; includes subdirectories named /Attachments/ (for storing email attachments, which may include .EMLXPART files) and /Messages/ (for storing .EMLX email messages); saves local copies of email messages from an IMAP account that has been setup in the software.

NOTE: IMAP mailboxes are actually folders and not files.