I reply to myself:
there is a bug in amavis configuration in etc/amavisd.conf.
On line 50 there is
@local_domains_maps = ( [".$mydomain"] );
the var $mydomain is initialized at line 20 and it is equal to domain name specified in cwp postfix build options.
I have manually pushed all my domain in the array in this way:
@local_domains_maps = ( [".mydomain1.it",".mydomain2.it",".mydomain3.it", ".myotherdomainetc.it"] );
now spamassassin works on all my domains.
My question now is if there is any way to specify all domains in the array without manually write them.
I have tryed with "*" but it doesn't work.