Jean-Francois Bilodeau wrote: : I run a mail server using a static IP, but for a while, GMail would : refuse any messages from my server, saying that it was a dynamic IP. : I've enabled SPF on my DNS server, and that seemed to fix the problem. : However, I'm curious to know how a server can know if an IP is static or : dynamic. How can you find out? There are people that manually maintain lists of dynamic IPs, presented via the traditional DNSBL mechanism. These lists are, as mentioned, manually maintained; there is no programmatic way to determine this information. (That's easier than 'very difficult', anyhow.) - Damian