solve "Connection could not be established with host smtp"?

centos_send_mail.txt

If your SELinux is in enforcing mode, you need to turn on httpd_can_sendmail and httpd_can_network_connect booleans.

You can verify if SELinux status is enforcing by running this command:

$ sestatus
...
Current mode: enforcing
...
Check status of httpd sendmail and network connect booleans:

$ getsebool httpd_can_sendmail httpd_can_network_connect
httpd_can_sendmail --> off
httpd_can_network_connect --> off
To enable sendmail and network connect and make changes persistant across reboots:

$ sudo setsebool -P httpd_can_sendmail 1
$ sudo setsebool -P httpd_can_network_connect 1