What you are implying in your reply is that Google ignores its own SPF validation results for spam purposes/mail acceptance then?
When you enable forwarding, even the emails that don't fail DKIM still show up in gmail with the SPF fail (under "Show Original"/"SPF"), and I'm sure they do in your setup as well since it's a logic consequence of doing the forwarding.
I'm essentially trying to figure out if any of these conditions can happen with forwarding enabled:
1) SPF validation will fail, the Google mail server will refuse the forwarded message, so the Gandi mail server will bounce it back to the sender without me even knowing, rather than depositing it in the associated mailbox (especially with the domains set with SPF "-all", such as several of the financial institutions I use). So, no POP3 fallback even if it's enabled along with the forwarding.
2) SPF validation will fail, the Google mail server will still accept the forwarded message, but will be more inclined to mark it at spam.
Thanks