Quoting Arjen de Korte
Then the recipients obviously use an ISP that does check SPF records *and* they use forwarding. In that case they have a problem with their ISP. You could have prevented their problems by publishing ?all in your SPF records and the e-mail should get through, even when they use forwarding. As an example, AOL and Amazon for instance do this, precisely to prevent the problems you're picturing.
Please explain the difference between the "all" record and not publishing SPF records period? If everyone sticks "all" in their SPF records, we have precisely the same situation we do now. Basically, SPF is just a "feel good" lark. In order to prevent it from breaking current e-mail, you have to break SPF. Then the SPF people say "Wowie! Look at all the people using SPF!" except that the biggest and probably most of the others use the "all" tag that says "everyone is considered trusted!" What's the point in publishing SPF if you publish that the entire internet is considered trusted for your domain? Oh, and I'm not "picturing" it. It actually happened to me. I was a big proponent of the idea of SPF until my customers started complaining.