Internal application emails out to Google mail domains rejected after migrating to Fortimail 400F

Hi there!

So my company recently migrated from our old Cisco C170 Ironport to a FortiMail 400F. Ever since migrating to the new appliance, all our internal applications' emails to Google hosted domains are getting bounced back, complaining that our emails are not RFC 5322 550-5.7.1 compliant and they go further to either say that the 'From' header is missing or there are multiple 'To' or 'Cc' header.

Emails sent from our users (desktop Outlook/webmail) to said Google hosted domains pass without issue. I'm half convinced that headers' manipulation is not working like how our Ironport did. May I know where can I check for said headers in the appliance gui?

Regards,

Ernest