It seems that emails sent by the forum are being flagged is unverified. 

Anyone had this issue before?

 

I've added DNS entry: v=spf1 mx a ptr ip4:XX.XXX.X.XXX ~all
(hid IP address)

Any thoughts?

Thanks,
Graeme

Do you mean Sender Verify Callout ? 

Managed to figure this out. 

Needed to update my DNS records in Google Domains, which I recently switched over to.

Thanks,
Graeme