IP addresses are an important tool for fighting spam, used for safe lists, blackhole lists, anti-spoofing and related purposes. While it is trivial to find the sender’s IP address in most email server software, it turns out to be surprisingly difficult to do so in email client software: we explain why. This implies that either alternative approaches are needed (typically signature-based) or that new standards for communicating between clients and servers are needed. We suggest several forms such standards might take.