Postfix will deliver to mailbox_transport as user 'nobody'. 
This is set by the option 'default_privs'.

Postfix is installed setuid root setgid dspam.
Add 'nobody' to /etc/mail/dspam/trusted.users

mailbox_transport = /usr/bin/dspam --user %u --deliver=innocent --mode=teft --feature=chained,noise,whitelist