Mailbox locking

[Darren Cook <darrenj@example.com>]

When I type "mail", then "q", I always get
 "Unable to lock mailbox: Permission denied"

This has been going on for a month or so (probably since a
Debian-auto-upgrade). It used to append mail to mbox in my home dir. If
I do it as root it works (but puts the mail in /root/mbox !!)

Here is my home directory:
drwxr-s---   15 darren   darren       4096 Apr 13 16:30 .

And here are the files in /var/spool/mail:
-rw-rw----    1 darren   mail        24957 Apr 20 08:01 darren
drwxrwsr-x    2 root     mail         4096 Apr 20 08:01 .

Permissions seem correct. Where else should I be looking for the cause?

Darren