‘MBox extension’ refers to a generic file format that is able to store email messages in simple text form.  ‘MBox’ is short for “mail box” and was a popular file format or extension because of its simplicity in the form of a text file.  When it comes to email or electronic mail messages, all data and information will be stored using a text file under the MBox extension or format.

What makes the MBox email extension or format very simple is that the data stored in the text file is stringed together or concatenated.  This simply means that all data in one electronic message, for example, will be stored side-by-side forming one long,  single line.  This formatting is done in order to save space and to take one line of data as a single item.  The basic structure of the MBox extension starts with the ‘From header’ of the email message.  This is typically followed by the sender’s email address.  Thereafter, all other strings or text are stored side-by-side and next to each other without spaces.  The simplicity of the Mbox extension and the corresponding text file format for data storage has become the most popular email data storage handler in the past.  With the text layout, processing data from various email messages can be handled easily and quickly.  There is also no concern in terms of format compatibility since most tools and applications are able to handle text formats readily.

The popularity of the MBox extension and format has resulted in different variants like: mboxo, mboxcl, mboxcl2, and mboxrd.  These formats were developed under the UNIX platform but have different specifications in terms of handling the email messages. Other variations of the MBox extension were also created to suit other applications and platforms including those of Eudora mail, various Mozilla applications, and Microsoft’s Entourage.