IMSVA would look at the received header when known host function is enabled. If foxproxy is not deployed at the edge of the organization, one cannot ensure that the SMTP client IP address is not modified. By introducing Known Host function, this problem can be resolved.

When Known Host is disabled, IMSVA will not analyze a received header except in one situation: when anti-spoofing filter is enabled. This filter validates that email sent from the internal email address was only processed by the internal mail servers. If "from" or "mail from" contains an internal address, anti-spoofing filter has to verify if all received headers only contain IP addresses which belong to the Trusted Internal IP List.