MBX is a mailbox file format commonly used to bundle multiple e-mail messages into a single text file. While historically .mbx files were mostly found on UNIX-like operating systems, they are now also supported by other messaging tools on Mac and PC.

Files with the MBX extension represent plain text files that store one email message after another. Each new message starts with the "From_" (from) line and ends with a blank line. There is also the message body between the "from_" line and the blank line.

The MBX file extension is also associated with mailbox files created by Microsoft Outlook Express. These files contain all messages that users send or receive through Outlook Express on that computer, including spam and junk messages.