Stop SELinux from blocking sendmail

You can check if SELinux is blocking sendmail by running:

/usr/sbin/getsebool httpd_can_sendmail

...which will report:

httpd_can_sendmail --> {on|off}

...and if it is off, you can turn it on using

sudo setsebool -P httpd_can_sendmail 1

with the "-P" for persistent (across reboots).