DMZ server hosting public IP address instead Fortigate (Proxy Based Deployment )

Hi All,

Based on the subject above, a customer would like to use all security features, especially Anti-Spam instead of deploying in transparent mode. 

However, they would like to maintain IP address holding by the server instead of Fortigate as one of their requirements. Appreciate if anyone can provide a knowledge base or link for the required configuration and design. Thanks