The Zimbra default is that any e-mail can have upto 1,000 recipients (total of TO/CC/BCC). I think this is a bit on the high side and can allow some spammer to highjack your mail server to do his devil's work (it happened to me). You can logon to a command line environment (Telnet, SSH etc.) and issue the following commands to set a lower limit. If the happy spammer then tries to add more than your limit to an e-mail and hit send, he will get an error saying he has too many recipients.

su - zimbra
postconf -e 'smtpd_recipient_limit = n'
zmcontrol stop
zmcontrol start

(n = the number of recipients.)