setup mail server

I am looking for someone to setup a mail server. I am using an Nginx reverse proxy to a remote server. I have tried James server because of its usage of Cassandra as a backend. I would prefer scalability but am open to traditional SQL servers. I have tried Mailu and cannot seem to get the certificates right. My frontend proxy has letsencrypt securing all traffic and then it needs to proxy to a server on my vpn and allow TLS.