DKIM, DMARC, SPF, rDNS/PTR, EHLO... all correct and verified in the email "original source," BY GOOGLE. Still goes to SPAM folder because **** you, apparently. I hate Google.
Another is most ISPs will put your IP in the PBL so unless you use a relay your outbound email is going nowhere. Google fiber was slow on updating their PBL when they rolled out service in my area so I got to send direct for several months before having to use an external relay.