Files with the .MBOX format contain e-mail messages sourced from the inbox, written in text form. A single text file can contain the entirety of the inbox – the messages are saved one after the other, with each beginning with a FROM headline.

Initially, MBOX files were used in UNIX-based e-mail clients – later on, they also became supported by other clients, such as Mozilla Thunderbird and Apple Mail, versions 1.X. The first inbox folder in the default version of the Apple Mail client for Mac OS was saved as INBOX.Mbox. Later versions of the Apple Mail software, namely versions 2.0 and newer, began utilizing a more convenient EMLX format, in which each message was stored in a separate file, which enabled easy searching with Spotlight.