Spf passes or fails depending on receiver address
Solution 1:
SPF is not failing. You're misinterpreting the results.
Google is doing the check (properly) when the mail is sent to the GMail account. Google is also adding the check when it retrieves via POP3. It doesn't know where it came from, therefore it marks a softfail.
I'm not sure why Google is checking for a POP3 retrieved email, but it shouldn't be.
You need to send it to an address that does it's own SPF check and get it directly, rather than let Google play with the headers.