This indicates an attack attempt against a Cross Site Scripting vulnerability in MailEnable Webmail.
MailEnable Professional, Enterprise & Premium 4.26 and earlier
MailEnable Professional, Enterprise & Premium 5.x before 5.53
MailEnable Professional, Enterprise & Premium 6.x before 6.03
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the latest version, available from the web site.