.IMAPMBOX File Extension
File TypeIMAP Mailbox
What is an IMAPMBOX file?
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.