Rather than start from scratch, build cake slices to test the theory out. Look at something like OPA, it provides a generic policy language with a pluggable framework. You could extend postfix if it is not supported and then implement the policies you described. Then look for market fit and pivot as necessary

I have nothing going on right now and looking for things to fill the void. Let’s start a google doc and build an mvp. Worst case, we have a GitHub repo with our names on it :D