FortiMail provides advanced, multi-layer protection against the full spectrum of email-borne threats
Article Id 190907
This article provides the CLI commands that are available on FortiMail 4.x and 5.x in order to clear statistics on the Statistics Summary page on a FortiMail device.

The following are some possible reasons for wishing to reset the statistics on the FortiMail web GUI:

a. to verify that the FortiMail is catching spam or viruses from a particular point (for example: after changes are made to the configuration)
b. redeployment of a device to a different site
c. after running tests before placing the unit into production

Run the following command on the console:

diagnose statistics clear

Sample command and output is as follows:

mail # diagnose statistics clear
System Time: 2015-03-13 20:29:27 EDT (Uptime: 25d 10h 29m)

*Note that the changes are immediate and will reset ALL of the counters.  At this time, there is no means to run a selective clear.