[exim] can exim generate a sha1 hash of each message ?

Hello,I have a requirement to ensure message integrity, starting from our server to the endpoint (which has no DKIM capability).
One approach I am thinking is to somehow make exim calls a shell command such as sha1sum that takes the *body* of the message,  get the hash value and insert into an x-header.
Does anyone know if this is possible ?Or do you have other suggestions ?
Thank you,
--luc