[exim] Exim + dovecot LDA problem

Hi there!

I have a problem with setting up delivery with the Dovecot LDA.

Exim is running under mail user.

Here is Exim configureation:

local_delivery:
driver = appendfile
file = /var/mail/$local_part
delivery_date_add
envelope_to_add
return_path_add

dovecot_delivery:
driver = pipe
command = /opt/dovecot/libexec/dovecot/deliver -c /etc/dovecot.conf
-d $local_part@$domain
message_prefix =
message_suffix =
delivery_date_add
envelope_to_add
return_path_add
log_output
user = mail

Exim log message:

2008-09-16 15:38:47 1Kfcdb-0003Ia-5W == <skipped> R=localuser
T=dovecot_delivery defer (13): Permission denied: failed to chdir to
/home/<skipped>

I think the problem is that delivery runs under mail user which hasn't
access to user home directory.
But under which user should LDA be running?

Thank you,
Sergey