Google's not particularly marking them invalid though? The not-being-authenticated indicator is just replacing any avatar with a question mark ("we're not sure

Google's not particularly marking them invalid though? The not-being-authenticated indicator is just replacing any avatar with a question mark ("we're not sure this person is this person") and a tiny indicator that warns you that outgoing mail is sent in the clear. As long as you're not sending mails to spam for this (which Google doesn't seem to be doing, unless of course a DMARC policy tells them to - I imagine) then I don't see the issue?