I had an extra Linode instance (Nanode 1GB: 1 CPU, 25GB Storage, 1GB RAM) laying around that I'd previously tried to manually set up a mailserver on. So I just reset it to the OS/version Mailinabox specifies, ran the install script and followed the rest of the instructions (https://mailinabox.email/guide.html)

It really doesn't need much, and the guide is very complete. Took me a Thursday evening plus some waiting for DNS propagation.