If they set "v=spf1 ip4:210.86.15.0/24 -all" then some mail (for example that trademe.co.nz sends out) would get rejected as it would be coming from the wrong place
This was a little unclear what I said . trademe.co.nz sends out emails sometimes from the email address of the trademe user. If a user has email address of customer(a)xtra.co.nz it will send from that address from trademe mail server. and if tested for SPF with a -all will fail and get rejected.
I can see SPF being a big learning curve to the NZ ISP customer base, once ISP's start enforcing SPF. We still see lots of email from customers who have an ispa email set as their email address but are using our email server because they moved ISP's but didn't want to change email address. I would be surprised if any ISP's don't see this. SPF seems like a very good idea, and will work perfectly for personal domains, but for big ISP's, xtra/clear/ihug etc. It's going to cause a huge influx of unhappy customers if it ever gets implemented. - Simon