Mail.app as of Jaguar can no longer "read" UNIX mboxes (well, that's what they want you to believe anyhow). Your options are:[list=a][*]Set up a POP server so Mail.app can read your target mbox (probably more work that it's worth),[*]Redirect the eMail to a different eMail address (or supress it if you're not interested in the output),[*]Mail.app uses mboxes for it's internal storage, so you could create a mailbox in Mail.app, and replace it with hard link to your other mbox (although this will take some fiddling to get it to work properly).[/list=a]
Personally, i think B is your best bet.