Sender Ip is always NULL about spamscope HOT 2 CLOSED

Extract a reliable sender IP address heuristically for each message.
        Although the message format dictates a chain of relaying IP
        addresses in each message, a malicious relay can easily alter that.
        Therefore we cannot simply take the first IP in
        the chain. Instead, our method is as follows.
        First we trust the sender IP reported by our mail server in the
        Received headers, and if the previous relay IP address is on our trust
        list (e.g. other well-known mail services), we continue to
        follow the previous Received line, till we reach the first unrecognized
        IP address in the email header.
        From article Characterizing Botnets from Email Spam Records:
            Li Zhuang, J. D. Tygar