Halon SMTP software supports the next generation IP protocol (IPv6), and has probably the best IPv6 support among commercial anti-spam products. Messages can be sent and received over IPv6 and it can by nature also act as a gateway betweens IPv6 and IPv4 messaging systems. IPv6 may coexist with IPv4 addresses. IPv6 addresses are treated no differently than IPv4 addresses. In many client applications however, you need to enclose IPv6 addresses in brackets eg. http://[2001:....]/. The only component that requires IPv4 internet connectivity (prevents it from running in a IPv6-only network) is the anti-spam real-time outbreak detection.