encrypt outgoing messages in the exchange server

dears

I am not expert in exchange and i need help from you in this case 

we use exchange server and we got incident that one of outgoing messages get compromised . 

whats your ideas to encrypt the outgoing from server side ?

any proper - post - guideline for setup the outgoing encryption  ?

so i can refer it to the tech person as he is new and our expert just left before the incident