Smail can no longer write mail files - what gives?

[Keilaron]

I've recently upgraded from Knoppix 3.7, I believe, to 3.8.1.
Everything went along smoothly (Great work ), until I tried sending/receiving mail.
My popd apparently no longer works properly, and smail no longer can write mail files.
What I mean is this: I write an e-mail, as root, to root...

# mail root
Subject: Test
This is a test e-mail.
Cc:

And I immediately get this message:

# smail: 04/24/2005 17:50:11: [12545] [m1DPozT-0000YKC] mail moved to /var/mail/error/1DPozT-0000YKC

So I did this (I've changed the @ sign so that phpBB doesn't turn it into an e-mail link, you know, spam and all):

cat /var/mail/error/1DPozT-0000YKC
<truncated>
To: postmaster [-AT-] keilaron.kicks-ass.org
Subject: mail failed, sending to postmaster
Summary: message returned due to delivery errors.
Reference: <m1DPozT-0000YHC [-AT-] keilaron.kicks-ass.org>
|------------------------- Message log follows: -------------------------|
 Xfail: <keilaron> parent: <root> reason: (ERR133) transport local: failed to open output file: Permission denied
|------------------------- Failed addresses follow: ---------------------|
 address: keilaron ... failed: transport local: failed to open output file: Permission denied
|------------------------- Message text follows: ------------------------|
<truncated>

What I don't understand is that smail is running as root and shouldn't be getting these errors. (The user "keilaron" here is the final recipient due to aliases.)
Root has no problems touching new files into where the mail file should go... so what's the problem?