If an E-mail containing an excessively long To: field in the header (~1.5 MB) is processed by First Class Intranet Services (FCIS), a Denial of Service can occur.
A remotely accessible overflow exists in Open Text's FirstClass mail server. The mail server fails to handle large e-mail to: values, resulting in an overflow. With a specially crafted very large request, an attacker can cause a denial of service resulting in a loss of availability.
Upgrade to version 6.1 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.