Hybrid Exchange traffic and Starttls

we need to forward traffic from Exchange Online to the Exchange on premise server.

direct connexion thru the firewall is working.

We wanted to use the F5 to loadbalance the trafic on the 2 on premise exchange server.

It seem that is a speciality with the starttls smtp trafic to do on the tcp port 25 😃. 

Thanks for your help

Daniel