That's the renamed copy of your original inbox.dbx file. So the 2 steps (both elementary Windows use) are:
1. Restore from the recycle bin
2. Rename inbox.bat to inbox.dbx.
If there already is a new inbox.dbx delete that first (that's elementary use of Windows also). If it already contains newer messages you don't want to lose, move them to another folder before you delete it (that's done in Outlook Express).