Well that is a typical spam to be honest ... Check to ensure that the networks listed are only yours for the MTA
Code:
su - zimbra
zmprov gs `zmhostname` zimbraMtaMyNetworks
You could also perform a remote test aswell
Mail relay testing.
With respect to reducing the amount of SPAM then I presume you have read
Improving Anti-spam system - Zimbra :: Wiki ? You may also wish to search for the forums for SaneSecurity.