[exim] /var/mail/xxx may not be a link

hi,
i have a 2-node tiny-cluster. one active, one passive node.
i need /var/mail/root available all time on both nodes but all the other
files should live on mirror

ln -s /mirror/var/mail /var/mail
worked so far.

but
ln -s /mirror/var/mail/xxx /var/mail/xxx
for every user xxx except root, does not, cause exim claims /var/mail/xxx
to be a regular file.

thanks a lot for any hint.
andreas