Sorry for your spam issues, I know how frustrating this can be. Some helpful steps to follow for an alternative method for blocking spam from coming into your email would be:
Click on the Preferences tab.
Click on Mail on the left side.
Scroll all the way down to Spam Mail Options.
You now have the option to Block Messages From or Allow messages from
Click Save and Refresh your screen.
Here is a link to the Zimbra website support tutorial page to assist with targeting these types of emails so they don’t re-appear again and get recognized as spam. We are working with our engineers to find out better resolutions to configure these email blocks If you have any further questions or concerns please give us a call at 866.945.3258
Your only other option would be using another email account such as google or yahoo which actually may have a better spam block then Zimbra may have concerning your emails.