Ok, let's try:
chmod -R 775 /var/spool/clientmqueue
chown -R root:mail /var/spool/clientmqueue
vi /etc/group
Make your user in mail group, e.g.:
mail::12:mail,your_user_here
Edit: I just took a look at mine and it can also be:
chmod -R 770 /var/spool/clientmqueue
Maybe it is more secure?
Edit2: You could also make it equal to mine:
chmod -R 770 /var/spool/clientmqueue
chown -R smmsp:smmsp /var/spool/clientmqueue
vi /etc/group
smmsp::25:smmsp,your_user_here
It is up to you!