Yes, you need to buy one or get one from one of the free CAs like StartSSL.... It needs to match your fortimail domain name.
Install it under System ---> Certificate ---> Local Certificate
Thank you Bromont. I looked at Start but they want my personal information to sign up. I don't particularly want to use my personal information for a business related product. Do you know of any other FREE SSL certificates?
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.