You could completely host it yourself (like running all the software) and use a spamfilter.
Not sure how @Tutanota handles spam on custom domains or use the domain via #addyio that's how I do it and it rocks (for me).
Just two remarks, some of jour points read like they are based on your assumptions. It's strange to write about 'Linux' because Linux is like an ecosystem of distros, which also are the core of qubesOS.
@cheprofumo
@MicrowavedTea
You could completely host it yourself (like running all the software) and use a spamfilter.
Not sure how @Tutanota handles spam on custom domains or use the domain via #addyio that's how I do it and it rocks (for me).