Fix a couple of typos
Add recipient groups
Add instructions for bounce message handling
Bit more info on what you should see on the happy path.
Reference Postfix as well as Exim - it's easier to setup.
Configure postfix non-interactively

And add the hostname to the domains to be delivered locally.
Don't focus on Ubuntu

Where Ubuntu and Debian differ, give both details.
Highlight path and user name
Link config vars.
Use the example domain, not localhost