The full form of MBOX is MailBOX. It’s used on Computing ,File Extensions in Worldwide

A mailbox is the storage location of electronic mail messages found on a remote server or downloaded to the user’s hard drive. Software e-mail programs commonly divide the mailbox into separate folders, notably the inbox, outbox, sent items, and deleted items.

An area in memory or on a storage device where e-mail is placed. In e-mail systems, each userhas a private mailbox. When the user receives e-mail, the mail system automatically puts it in the mailbox.

The mail system allows you to scan mail that is in your mailbox, copy it to a file, delete it, print it, or forward it to another user. If you want to save mail, it is a good idea to copy it to a file, because files tend to be more stable than mailboxes.