If you want then to cache your content to reduce the load of your servers, they have to decrypt the traffic. This is how a reverse proxy works.
And, well, you have to trust them before contract their services. The same way people trust vpns to route their traffic. If I was from some 3 letter agency and want to spy on potential illegal content, I would tap into a vpn server.
I used to manage an Exchange 2000 Server. I don't miss this.