OP might still have some configuration issue (check the headers of the mail that has been placed in the spam folder. Gmail usually tells you what the problem is

OP might still have some configuration issue (check the headers of the mail that has been placed in the spam folder. Gmail usually tells you what the problem is). Last August, I have added IPv6 to our mail server, so its address for sure didn't have any prior reputation. Once the PTR and SPF records were correct, sending mail over v6 to gmail was no problem at all. I have yet to see any delivery issue over v6 that's caused by a lack of reputation