Enabling SMTP TLS End to End

Hello -

I have a requirement to enable SMTP TLS communication all the way thru the backend SMTP gateways. My question is we are terminating the client side SMTP TLS on the F5 VIP, what irule can I use to extend the TLS communication to the backend nodes so it not in clear text? We still want the F5 VIP to host the public wildcard certificate but the backend nodes will be using self-signed certs. Do you have to enable the server side profile on the VIP? If so, how do you also write the irule to where if doesn't detect STARTTLS then do not encrypt to the backend?