how can I prevent zimbra to send email as root user?

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http://serverfault.com – zimbra is working on ubuntu 12.04 and there are some system users ie. root. zimbra is trying to send email to itself [email protected] and hangs on mailq then prevents other mail's distribution. but I cannot stop it to email. I tried to use aliases file as root: /dev/null but does not work. what else can be done to do prevent this? (HowTos)