Really stupid issue with permissions on CentOS 5.1 and real easy fix, since there were improper permissions on the mqueue and clientmqueue directories.
The Error message:
Tried to send mail via command line using ‘mail’ command, and it returned this error:
can not chdir(/var/spool/clientmqueue/): Permission denied
Program mode requires special privileges, e.g., root or TrustedUser.
The fix is to set permissions on this directory to that of the submission user (smmsp) on CentOS.
chown smmsp:smmsp /var/spool/mqueue/
chown smmsp:smmsp /var/spool/clientmqueue/
Now it works. Hoorah.