Thunderbird is the Mozilla mail client. It comes in multiple languages and versions for multiple operating systems (Windows, OS X, Linux).
Each mail folder (Inbox, Sent, etc.) is stored as two files; one with no extension (e.g. INBOX), which is the mail file itself (in ‘mbox’ format), and one with an .MSF extension (e.g. INBOX.MSF), which is the index (Mail Summary File) to the mail file. If importing to another program, tell the other program to import mail from the file with no extension.