ok it only took a few minutes on wikipedia to get a handle on the "v=..." part.
That SPF record says that mail is allowed to be sent from the "A" and "MX" systems, and all others should be forbidden.
So does this mean that the IP of the "sender's address" doesn't match the IP in the A record? And that's why the message is bouncing?
I'm sending my mail through the SMTP defined as "mail.(mydomain).com", authenticated by password.
The DNS record for the "mail" subdomain is a CNAME to (mydomain).com.
The MX record for (mydomain).com is  (mydomain).com.
The A record for (mydomain).com is the IP address of my server.
As far as I can tell, everything's set up properly. And... it's worked fine until just a few days ago when I started getting occasional messages bouncing back.