[Exim] Classic lock file hitching post problem

Hi folks,

I'm getting the classic "lock file hitching post" errors that I've
helped other people with in the past, only I'm absolutely stumped for an
answer this time.

I'm using exim-3.20.

Check this out:

/usr/local/etc/exim/configure:
==============================

exim_user = sendmail
exim_group = mail
never_users = sendmail : root

[...]

local_delivery:
driver = appendfile
file = /var/mail/${local_part}
delivery_date_add
envelope_to_add
return_path_add
group = mail
mode = 0660

Permissions and ownerships on /var/mail:
========================================

ls -ld /var/mail
drwxrwxr-x 2 root mail 512 Mar 4 12:12 /var/mail

I've even tried adding ``user = ${local_part}'' to the local_delivery
configuration. Regardless, I get these error messages:

2001-03-04 12:12:56 14ZVVY-0000wZ-00 == /var/mail/arbuser <arbuser@???> T=address_file defer (13): Permission denied: creating lock file hitching post /var/mail/arbuser.lock.axl.fw.uunet.co.za.3aa21528.00000e31

I've also checked the ChangeLog for exim-3.22 and can't see any fixes
that might relate to the problem.

Any help would be greatly appreciated.

Ciao,
Sheldon.