[exim] exim dovecot sieve integration

Hello all.

I have just a generic question. Exim delivers fine to dovecot-lda. My
problems are the logfiles. Whenever I recieve a message
Exim writes to dovecot.log as the user who recieves the mail. This
forces me to have that logfile world writeable. I don't
really like that. How do you guys deal with that? Here my transport:

dovecot_delivery:
driver = pipe

# You may or may not want to add -d $local_part@$domain depending on
if you need a userdb lookup done.
command = /usr/libexec/dovecot/dovecot-lda -f $sender_address

message_prefix =
message_suffix =
log_output
delivery_date_add
envelope_to_add
return_path_add
#group = mail
#mode = 0660
temp_errors = 64 : 69 : 70: 71 : 72 : 73 : 74 : 75 : 78

thanks and best regards
t.