Sorry for the late response.
I had those configurations for postfix and courier, to use .maildir, in place already. After all, I'm using .maildir for some time now.
The reason I thought the mail was delivered to /var/mail, was that mutt was using this dir, by default. That set me on the wrong track. The actual reason the mail wasn't delivered, was that there was no .procmail file present in the home directory. :r