On 29 Jul 2023, at 20:07, Hugo Mills via BitFolk Users <users@mailman.bitfolk.com> wrote:

  However, I think I have the right TXT record in the DNS for carfax.org.uk:

@    TXT     "v=spf1 mx a ip4: ip6:2001:ba8:1f1:f0e6::/64 a:mail.carfax.org.uk a:savella.carfax.org.uk -all"

and the diagnostic message from gmail isn't all that helpful about why
it's not matching.

  Does anyone have any idea what I've missed here?

You also need a TXT record for savella.carfax.org.uk, e.g.

“v=spf1 a -all”

That’s what the recipient SMTP server will be looking for.