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