This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in MailStore Server.
The vulnerability is due improper validation when the affected application handles a maliciously crafted request. An attacker can exploit this by tricking an unsuspecting user into clicking a malicious crafted link and execute arbitrary code within the context of the targeted users' browser.
MailStore Software MailStore Server 9.0 to 10.0.1
System Compromise: Remote attackers can execute arbitrary script code within the context of the target user's browser
Apply the most recent upgrade or patch from the vendor