One potential option is using CloudFlare email routing. You can use it with a regular public provider (gmail for example). CloudFlare will handle the MX side of things for your custom domain and route your emails to the public provider without requiring it to use the custom domain. The issue I’ve found is that it’s hard to set up a reply address to reply as the original email was received, so this may not work perfectly (emails look like they come from the gmail address for example). Does anyone know how to solve that part?