Had the same problem. Combed the conf dir for string 6.6 and guess what? It's defined in amavisd.conf:
$sa_tag2_level_deflt = 6.6; # add 'spam detected' headers at that level
Cheers.
Martin
update: it's more complicated. amavisd.conf seems to be generated from amavisd.conf.in while the daemon is (re)started. amavisd.conf.in defines the line like this:
$sa_tag2_level_deflt = %%range VAR:zimbraSpamTagPercent 0 20%%; # add 'spam detected' headers at that level
This variable can be set in the admin interface under Global Settings AS/AV tab. So, in the end, it's less complicated than I thought
Default value is 33% (for crying out loud, why convert SA points to %?) which yields 6.6, so 100% is probably 20 (rounding down). If you want a spamassassin required_score of 5.0 you should fill in 25%.
I'm speeking for Zimbra 5.0RC2, btw.