I have a domain cabritacale.eu hosted on Google, and I have set the
TXT type DNS register for that domain similar with GMail's.

Any email I send from my acount @gmail.com to a third domain passes
verification with SVE when read in Thunderbird.
"The sender was explicitly permited by <_spf.google.com> with SPF."

Any email I send from my account @cabritacale.eu to the same third
domain fails verification with SVE when read in Thunderbird. The SVE
message is:
"This does not appear to be a legitimate <cabritacale.eu> email."
"The sender was not permited by <cabritacale.eu> with SPF."

I analized the headers for both emails and they exit Google through
the same server: ug-out-1314.google.com ([66.249.92.171]).

If the origin server is the same and the TXT record is the same for
both domains why is it that one passes and the other fails?