The ability to delegate a subdomain out to an SMTP server hosted by an app has been discussed several times before. I'd quite like to package Mailman for Cloudron, but need a way of getting mail into Mailman itself. Mailman processes incoming mail via LMTP, but we could run an MTA on the app container if the app didn't support native SMTP. If we went that route I think it would be essential for Haraka to call forward to the SMTP server running in the container to avoid backscatter. I do run Mailman on my Cloudron instance for now in a completely unsupported manor and allow incoming SMTP via the Cloudron firewall as a custom port. This is working fine with Exim installed natively on the host, and I have a script to ensure the Nginx config for Mailman is in place. I really wouldn't recommend doing this yourself though and I'm also thinking of offloading the mailing lists to a third party provider at this point for various reasons.