Sendmail chdir(/clientmqueue) permission denied

Solution 1:

Sendmail-8.12+ installation: file permissions

Sendmail-8.12+ binary should be installed as set GROUP id.
(/usr/sbin/sendmail.sendmail in your case)

It is described in SECURITY file in sendmail(.org) distribution:

-r-xr-sr-x  root   smmsp    ... /PATH/TO/sendmail
drwxrwx---  smmsp  smmsp    ... /var/spool/clientmqueue

Solution 2:

There's also an SELINUX boolean policy called httpd_can_sendmail.... I'm pretty sure this has to be set to true also.