SPAM generated by Zimbra Server

[pete irvine]

Jul 26 16:06:35 sane-dog postfix/smtpd[32191]: NOQUEUE: reject: RCPT

from xxx.xxx.co.nz[xxx.xx.xx.xx]: 450

: Recipient address rejected:

Domain not found; from=

to= proto=ESMTP

helo=
1. How do I stop this repeatitivly sending

2. How do I stop this happening again.
Thanx