I did some more testing yesterday.
@davidkingham - DKIM and SPF are 100% set on all my domains so that can’t be the issue.
I’ve tested several Gmail boxes and even tried to change the domain - for example instead of domain.com we have forum.domain.com for our discourse. That didn’t seem to make a difference.
@codinghorror our mail settings are set correctly as far as I’m aware, we don’t have an issue with spam but rather with Google flagging almost every mail as promotions.
We do send promotional emails, but obviously our discourse forum isn’t promotional but rather updates (digest) or private messages.
Not sure what to do really.