Getting "This message does not meet IPv6 sending guidelines"

[avij]

ip6:2606:400:0:5:240:f4ff:fe04:c83c/64 is a little bit incorrect. You should use either ip6:2606:400:0:5::/64 to cover your IPv6 address range (depending on your prefix length, I just assumed a /64), or the full IPv6 address ip6:2606:400:0:5:240:f4ff:fe04:c83c.

Technically, what your ISP would need to add aren't AAAA records but PTR records, specifically c.3.8.c.4.0.e.f.f.f.4.f.0.4.2.0.5.0.0.0.0.0.0.0.0.0.4.0.6.0.6.2.ip6.arpa. IN PTR stargate.mehconsulting.com. You should let them know that the lack of such a record hinders your ability to send email to Google, and that you're not happy about the situation. If they still can't be bothered to add such a record, well ..

If the SPF adjustments aren't enough to make Google happy, I'm afraid your best bet would be to route your Google-destined emails via your ISP's mail server. Sendmail's mailertable feature might help with this. I'll leave the exact configuration details for someone else, as I'm not using sendmail myself. Perhaps this page helps. On the other hand, doing things that way would require further changes to your SPF configuration. The SPF TXT record would need to contain the information that your ISP's mail server can relay mail from you.

[avij]

Although this isn't directly related to your problem, I noticed that your MX record points to mail.mehconsulting.com, which is a CNAME to stargate.mehconsulting.com. MX records should not point to CNAMEs. The easiest way to fix this is to remove the "mail stargate.mehconsulting.com. CNAME (Alias)" configuration and add an A record 65.78.188.61 for mail. If your sendmail listened on the IPv6 address (which doesn't seem to be the case at the moment, or it may also be blocked by the firewall), you could also add AAAA 2606:400:0:5:240:f4ff:fe04:c83c for mail. For some background, read RFC 2181, section 10.3.

[MarkEHansen]

Thanks. I've made the changes you've recommended. I appreciate all the help you and the others have been providing.

[MarkEHansen]

I've asked my ISP to provide a PTR record to allow a reverse DNS on my IPv6 address. They said they would do this, but the tech made clear that the IPv6 address is not being provided by them. He said they are not doing anything with IPv6 for another few years at least, and that it is my machine that is creating this address.

Is this true? Is this IPv6 address configured on my CentOS machine somewhere? Can anyone tell me where that is? My concern is that if I should have to replace this machine with a new one, I'll need to configure the same IPv6 address so the externally-configured DNS records will still match.

I looked at /etc/sysconfig/network and see that NETWORKING_IPV6 is set to no.

Thanks,