@philandstuff: I’m also not an expert in these things
That IPv6 address does not look correct to me. None of the
*.dhall-lang.org services, including Discourse and the mail server, are configured to use IPv6 and I don’t believe that IPv6 address you listed is for
$ curl https://[2600:3c01:0:0:f03c:91ff:fe40:4ef3]/
curl: (7) Couldn't connect to server
$ host dhall-lang.org
dhall-lang.org has address 220.127.116.11
dhall-lang.org mail is handled by 10 mail.dhall-lang.org.
$ host discourse.dhall-lang.org
discourse.dhall-lang.org has address 18.104.22.168
discourse.dhall-lang.org mail is handled by 10 mail.dhall-lang.org.
$ host mail.dhall-lang.org
mail.dhall-lang.org has address 22.214.171.124
Do you have any idea why the source IP address might be wrong? It might be specific to your mail setup. As far as I can tell you should not be receiving mail from our Discourse with an IPv6 source address at all and
126.96.36.199 is the correct address to specify in the SPF record.