Get rid of ALAVETELI_USER and ALAVETELI_HOME

It's not clear whether they are actual variables (they aren't).
Fix Postfix configuration instructions.

Copy the file changes used by install_site.sh
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.
Use the example domain, not localhost