>If your work around is to use ip4 to send to us, that's fine. The >percentage of spam mail on ip6 is higher than on ip4. Our own outgoing >code makes choices on whether to use ip6 or ip4 based on expected >authentication level, and it is ugly. I don't like ugly, but I don't like >spam either. At the moment, my setup is identical for IPv4 and IPv6, with of course a preference for IPv6. If gmail would first return a 4xy then I would never notice, because my mail server would retry over IPv4. Unfortunately, every now and then gmail returns a 5xy for no reason on my side. Then I resend the same e-mail from my MUA and it gets accepted. I don't know why gmail seems to turn transient errors into permanent failures at the SMTP level. As far as I can tell, that is unique to gmail.