Re: "we offer IPv6 and nobody except Googlebot uses it."
"An IPv6-only mail server is not likely to be able to receive mail from most of the world, so what's the difference?"
If IPv6 were widely deployed, this would be a non-issue. Using it as an excuse not to deploy is a self-fulfilling prophecy.
Have you lot all got secret stacks of /16s that you want to (ugh!) "monetise" in the near future, or what?