The problem with running email server on mobile is that it will be unavailable whenever the user does not have a connection, making it impossible to deliver message in their inbox.
One more or less practical solution I see is to add support for .onion domains and a simple dockerized setup that allows to run such server without the problems of registering a domain, populating DNS records, getting a TLS certificate and so on. Such servers will normally be able to speak only to other .onion servers, but with multi-account support you can run such experimental DC server in addition to your main account.
See related topic: Delta chat through tor?