Exim4 & non standard user directories

[Alan Ianson <alianson@shaw.ca>]

Exim4 works great here for the usual users in the /home/$USER directories, but 
I have some users in different (/opt/mbse/home/$USER) non standard locations. 
These users can send mail without any problem, but when a message arrives 
here for them it is bounced after a few days with the explanation that we 
couldn't chdir to opt/mbse/home/$USER.

Those directories are owned by mbse:bbs, and are rwx by user and group but not 
others. What do I need to do so that exim4 can chdir to /opt/mbse/home/$USER?

Thanks in advance!