SELinux prevents Apache from sending emails on CentOS.
# sestatus SELinux status: enabled SELinuxfs mount: /selinux Current mode: enforcing Mode from config file: enforcing Policy version: 24 Policy from config file: targeted
# getsebool httpd_can_sendmail httpd_can_sendmail --> off
# setsebool -P httpd_can_sendmail 1
# getsebool httpd_can_sendmail httpd_can_sendmail --> on