In fact, sendmail doesn't write local mail files. If it's RedHat, then I'm guessing that procmail wrote it. I think they use that for their standard MDA.

And procmail uses Berkeley mbox files, unless I'm mistaken.

The only reference I see to doing this is to install Netscape 4 (specifically not Mozilla or Netscape > 4). Then copy the mbox file to Netscape's mail directory, where it will create its own index. Then Outlook is supposed to be able to import that.