I currently run email on ~10 domains using one $5/month digital ocean droplet running Mail In A Box. Adding a new domain is as simple as pointing nameservers to

I currently run email on ~10 domains using one $5/month digital ocean droplet running Mail In A Box. Adding a new domain is as simple as pointing nameservers to my server and creating a user with an email address on that domain (which can be done with curl using a simple one-liner api call, or manually through the admin interface) MIAB handles provisioning ssl certs, mta-sts, dane tsla, verifying we're not blacklisted at spamhaus, configuring roundcube and everything else. My domains get ~5k incoming messages a day. Maintenance litterally involves sshing into the server and running apt update/upgrade once a week or so and could easily be done as a cron job.