I think this is the wrong approach (self-hosting a mail server). Just use a dedicated service like MxRoute, Migadu, FastMail, StartMail, or similar. Heck, even Google Workspaces and Microsoft 365 are a better option. That’s my $0.02, anyway ;-)

It’s not a recommended topology (I/O perf & reliability), but if you do this, you’ll want to use a “block storage” protocol (e.g. iSCSI, etc.), and a very fast network (i.e. 10Gbps min, ideally 25 or 40).