There is only one file type that uses the .MBX extension:
Outlook Express Mailbox
Mailbox (or mail folder) that contains Outlook Express e-mail messages; contains actual message data; may be backed up by copying the mailbox files to a different folder.
MBX files may also be created by Eudora, an email client for Windows and Mac operating systems that is no longer developed. You may be able to open Eudora MBX files with the Thunderbird email program.
Updated: June 28, 2018