We can incresed spamscore 6.6 to 8 from zimbra admin console.I have verified in testzimbra server our localinstance and we can increase and decrease spamscore.
Default spam score 6.6:-
Login to Admin console of Zimbra---configure----Global settings
In Global settings select on AS/AV then i made Tag percent default 33% to 40%
Tagpercent = 20*40%=8
The Required score is 20 times the percentage in that tag field.
So for example, 20 * 40% (0.40) = 8.
After modified We need to restart zimbra service in command mode