Unable to Establish StartTLS Connection

Hello,

I have just installed on-premise exchange server and configured hybrid with office365. However, I noticed that I am not able to establish a StartTLS connection with office365 on port 25 and this causes email from on-premise user to a cloud user to fail. I have allowed all services on the firewall policy just to be sure that I am not blocking it but it still fails. I have attached a screenshot of failed connection test form behind fortigate and successful test from outside of Fortigate.