This indicates an attack attempt against a persistent Cross-Site Scripting (XSS) vulnerability in Zimbra Collaboration.
The vulnerability is caused due to insufficientl sanitizing of email body content. It allows remote attackers to launch XSS attack against Zimbra Collaboration users.
Zimbra Collaboration 8.6.0 Patch4 and before
System Compromise: Remote attackers can execute arbitrary script code in the context of the affected user.
Apply the vendor's patch which is available at