At my request, Jason H. sent our group's list research to the provider support people. They said there is one IP per server, and there's one U.S. server, so that issue is moot. Jason also told them: ------------------------------ We were seeing inconsistent spam flagging from some providers, the biggest of which was Google. We noticed that in some cases, incoming mail from www.mailmanlists.us to Google's SMTP servers were failing SPF validation. Looking into the headers of said messages showed two different scenarios. One where SMTP communication occurred over ipv4, which was fine. And one where it occurred over ipv6 which resulted in the following header: Received-SPF: softfail (google.com: domain of transitioning vcf-midatlantic-bounces@lists.vintagecomputerfederation.org does not designate 2604:3400:dc1:43:216:3eff:fe36:d305 as permitted sender) client-ip=2604:3400:dc1:43:216:3eff:fe36:d305; Adding your ipv6 address (2604:3400:dc1:43:216:3eff:fe36:d305) to our SPF record has cleared up the random spam flagging issue, but does raise a couple of other issues. Many times reverse lookups are used to help identify spam, since you don't have an AAAA record defined for www.mailmanlists.us, nor a reverse ptr record, this may cause an issue for some clients. Though we've not witnessed any adverse behavior attributable to this, it is something to keep in mind. A few suggestions on how to fix the issue for all your customers who may unknowingly be experiencing issues: 1) Either kill ipv6 on your host (which is probably not the right way forward given the way things are moving). or 2) Get your ipv6 entries setup, and have your clients add analogous ipv6 entries for their lists DNS and SPF records. Honestly, don't feel bad about this, I think google made some change a couple weeks ago and I was caught flat-footed on this with my own mail server when everyone suddenly started talking ipv6 to each other. ------------------------------ List support guy wrote back: "We'll follow your suggestions and get them done this afternoon." ------------------------------ That is amazing. - Evan