is the fully-qualified domain name (FQDN) of the Exchange server is either the FQDN or the IP address of the server that initiated the connection. The FQDN would be used if the Exchange5.5 IMC is able to resolve this information through a reverse DNS lookup; the IP address would be used if a reverse DNS lookup was not possible or failed to resolve the connecting servers IP address.
It is important to note that the attacker could not simply send data to the IMC in order to overrun the buffer. Instead, the attacker would need to create a set of conditions that would cause the IMC to overrun its own buffer when it generated the EHLO response. Specifically, the attacker would need to ensure that a reverse DNS lookup would not only succeed, but would provide an FQDN whose length was sufficient to result in the buffer overrun.