$ echo test | mail -s "test" to@example.com -c cc@example.com
WARNING: RunAsUser for MSP ignored, check group ids (egid=102, want=51)
can not write to queue directory /var/spool/clientmqueue/ (RunAsGid=102, required=51): Permission denied

# BAD
$ echo test | mail to@example.com -s "test"
# BAD
$ echo test | mail to@example.com -c cc@example.com
# GOOD
$ echo test | mail -s "test" -c cc@example.com to@example.com