blocking bcc mails

[fathianf]

I would like to block emails that don't have "To" address and all addresses are in bcc field. please let me know how to do this with zimbra 4.5.7

[mmorse]

Both spamassassin rule or postfix header check methods outlined here: Is it possible to give "undisclosed-recipients" a positive spam score?

(The OP seems to have had trouble getting the header check method to work.)

(You should seriously consider upgrading to 4.5.11 then 5.0.18)