I have managed to get Mail.app to handle sendmail mail by cheating... I am assuming that you're running sendmail OK but have been using mail or pine or somesuch to read mail.
Do this:
in the terminal, copy the folder heirarchy of another of your mailboxes (in ~/Library/Mail/Mailboxes).
Switch into the newly created directory, and instead of mbox, do:
ln -s /var/mail/user_name mbox
where user_name is your username. Restart Mail.app and you should now be able to manage mail downloaded from sendmail in Mail.app
If anyone knows a better way, preferably from within Mail.app instead of silly hacking, please let me know!!!
Cheers,
Geoff Saulnier - Mac, *NIX, perl, hack!!
[Editor's note: I'm not running sendmail locally, so I have not tried this myself]

