My Company is using ZCS paid version.
Last time we had experienced serious duplicate emails (reached 40k and keep sending) to recipients.
What Zimbra support told me is to upgrade the ZCO to the newest version.
After few months later, the same problem re-occurring but only 2 to 5 emails duplicated sent our recipient.
I've Contacted Zimbra support and they ask me to upgrade the ZCO again but at the end the problem still there.
The support then ask me to recreate the profile and delete the ZDB, everything I've done and still not solve the issue.
Now I got nothing update from support team.
I feel that Zimbra sucks when I first look at this immature product and had so many compatibilities issue.