Thanks for the clarification. When Clearcenter allow me the time, I'll update the HowTo, but note that your comment applies to which ever site hosts your domain. It is not restricted to Clearcenter hosting your domain.

FWIW your SPF record can also be simplified from:to:as hardingclan.org is assumed for the A and MX records. Your A record directly resolves to your IP and your MX record resolves to mail.hardingclan.org which resolves to hardingclan.org so there is no point in specifying the IP and the include just sends the SPF lookup in circles.