Thanks to http://david.schweikert.ch/, a small but powerful utility to give insights on your postfix mail server
Mailgraph is postfix logging tool. Install mailgraph
For ubuntu follow the steps
sudo apt-get install rrdtool mailgraph
During the installation, the system startup links for Mailgraph are created automatically, and Mailgraph also gets started automatically, so we don’t need to start it manually.
Mailgraph is now accessible trough the hostname or IP of your server. Open a web Browser and enter the URL http://192.168.1.100/cgi-bin/mailgraph.cgi or http://server1.example.com/cgi-bin/mailgraph.cgi, and you should see some graphs. Of course, there must be some emails going through your system before you see the first results, so be patient.
On Centos 7
yum install rrdtool mailgraph
Deny from all
Allow from all
restart httpd service