Mailgraph is a very simple mail statistics RRDtool frontend for Postfix and Sendmail that produces daily, weekly, monthly and yearly graphs of received/sent and bounced/rejected mail.
Assuming you already have Postfix and Apache installed, just install these packages:
sudo apt install rrdtool mailgraph
Navigate to this location to view the graphs:
http://yourserver/cgi-bin/mailgraph.cgi