opnsense-ports/mail/mailgraph/pkg-message

Edit /etc/rc.conf and set

  mailgraph_enable="YES"

to enable mailgraph. 

Make sure this script should be invoked with privilege to read
/var/log/maillog. For example, you may change the permission of
maillog to be 644. Another way is to make maillog owned by
the mail group, and put mailgraph_user in that group.