To implement security for the traffic between two VPCs in AWS, while keeping separate management of each department's VPC, two possible actions are:
Create a transit VPC with a FortiGate HA cluster, connect to the other two using VPC peering, and use routing tables to force traffic through the FortiGate cluster. This option allows the cybersecurity department to manage the transit VPC and apply security policies on the FortiGate cluster, while the other departments can manage their own VPCs and instances. The VPC peering connections enable direct communication between the VPCs without using public IPs or gateways. The routing tables can be configured to direct all inter-VPC traffic to the transit VPC.
Create a VPC with a FortiGate auto-scaling group with a Transit Gateway attached to the three VPCs to force routing through the FortiGate cluster. This option also allows the cybersecurity department to manage the security VPC and apply security policies on the FortiGate cluster, while the other departments can manage their own VPCs and instances. The Transit Gateway acts as a network hub that connects multiple VPCs and on-premises networks. The routing tables can be configured to direct all inter-VPC traffic to the security VPC. References: https://docs.fortinet.com/document/fortigate-public-cloud/7.2.0/aws-administration-guide/506140/connecting-a-local-fortigate-to-an-aws-vpc-vpn https://docs.fortinet.com/document/fortigate-public-cloud/7.0.0/sd-wan-architecture-for-enterprise/166334/sd-wan-configuration

a) The FortiMail access control rule to relay from Office 365 servers FQDN is missing.
If the access control rule to relay from Office 365 servers FQDN is missing, then FortiMail will not be able to send emails to Office 365. This is because the access control rule specifies which IP addresses or domains are allowed to relay emails through FortiMail.
b) A Mail Flow connector from the Exchange Admin Center has not been set properly to the FortiMail Cloud FQDN.
If the Mail Flow connector from the Exchange Admin Center is not set properly to the FortiMail Cloud FQDN, then Office 365 will not be able to send emails to FortiMail. This is because the Mail Flow connector specifies which SMTP server is used to send emails to external recipients.

The Jinja template will not automatically map the interface with "WAN" role on the managed FortiGate. The administrator must first manually map the interface for each device with a meta field.
The template will work if you change the variable format to {{ WAN }}. The {{ }} syntax is used to define a variable in a Jinja template.