Updated: March 07, 2025

.MBOX File Extension

There is only one filetype that uses .MBOX file extension and it is Email Mailbox.

Email Mailbox

File category:	Text Files
Developer:	N/A
Open with:	Microsoft Outlook (+9 more)
Platforms:	Windows, macOS, Linux, iOS, Android, ChromeOS, iPadOS

An MBOX file is an email mailbox format that stores multiple email messages in a single text file. It's a container that holds a collection of electronic mail messages along with their attachments. In this format, emails are arranged consecutively, with each message starting with a header section (beginning with "From") and followed by the message body.

The MBOX format existed for many years without standardization. It wasn't until 2005 that it was formally standardized as RFC 4155, defining the application/mbox format. Before this standardization, it was primarily used in Unix systems.

MBOX files serve several purposes in email management:

Storage and organization: They store entire inboxes or mailboxes in a single file, where new messages are added to the end of the file.
Email client compatibility: Although originally used exclusively by Unix hosts, MBOX is now supported by numerous email applications including Mozilla Thunderbird and Apple Mail.
Email export and backup: Apple Mail exports mailboxes as MBOX files when users right-click a mailbox and select "Export Mailbox".
Data migration: Users can transfer emails between different email clients using the MBOX format.

Earlier versions of Apple Mail (version 1.x) used MBOX files stored in the /User/Library/Mail/Mailboxes/ directory, though newer versions have moved to the .EMLX format for individual messages to enable Spotlight searching.

How to Open .MBOX as Email Mailbox

There are several ways to access the contents of an MBOX file:

Text editors: Since MBOX files are plain text files, you can open them with any text editor such as Microsoft Notepad, Apple TextEdit, or GitHub Atom to view their raw contents.
Email clients:
- Apple Mail: Select File → Import Mailboxes and follow the prompts. Imported messages will appear in the Import mailbox.
- Mozilla Thunderbird: Install the ImportExportTools NG add-on, then select Tools → ImportExportTools → Import MBOX File and follow the prompts. Messages will appear in the Local Folders inbox.
Conversion utilities: If you need to use the emails in Microsoft Outlook, several tools can convert MBOX to PST format, including Regain MBOX to PST Converter, EdbMails, and Aryson MBOX to PST Converter.

It's worth noting that MBOX files cannot be easily opened on mobile devices. While the format is plainly structured, mobile environments typically don't support direct viewing of MBOX containers.

When reading an MBOX file, applications scan for "From_" lines that mark the beginning of each message. The reader extracts the sender information and delivery date from this line, then reads until the next "From_" line or end of file, processing the content as an RFC 822 message.