[exim] Exim umask/permissions?

Hey Folks,

I hoping this is a pretty simple question:

I'm running Exim+Courier-IMAP. Whenever Exim delivers a mail to a
domain that it hasn't seen before, it creates the appropriate maildir.
(ie, domain/user.) However, the permissions on the maildir that it's
creating are too restrictive:

mailserv# pwd
/home/maildir/mjeungtestdomain.com

mailserv# ls -alht
total 288
drwx------     5 mailnull  mail      1K Apr 10 16:26 mjeung         
<--------------  700
drwx------     3 mailnull  mail     80B Apr 10 16:26 .                 
<--------------  700
drwxr-xr-x  4586 mailnull  mail    266K Apr 10 16:26 ..

I want to create the directory and all files below it so they are
group-readable...

My delivery router looks like this:

cis_local_delivery:
driver = appendfile
maildir_format=true
maildir_tag= :S=$message_size
quota_size_regex=S=(\d+)$
directory = /home/maildir/$domain/$local_part/
delivery_date_add
envelope_to_add
return_path_add
group = mail
mode = 0660

However, it doesn't seem to be respecting the mode=0660 directive.
The folder is being created with permissions of 700, which is too
restrictive.
How do I make exim create directories with 755 permission?

Thanks!
Michael Jeung