this is very strange and i was not able to take it seriously until i see it grepping zimbra.log
the short story:
i add ~20 adresses in and restarted zimbra; i tried to send an email from one of them and it was blocked (blacklist_from
then one other of them send 2 mails, one was blocked (then the filter is ok and it works fine); the second one, instead, PASS

the first mail
Oct 3 13:51:38 mailz amavis[26876]: (26876-08) Blocked SPAM, [] [] <> -> <>, Message-ID: <>, mail_id: KLXUg0TW-tMp, Hits: 99.824, 3922 ms

the second
Oct 3 13:51:41 mailz amavis[25924]: (25924-09) Passed CLEAN, [] [] <> -> <>, Message-ID: <>, mail_id: ck8U+WKxUSYZ, Hits: -, queued_as: 5642E3A4003, 1446 ms

incredible but true
any idea?