Add instructions for installing postfix.
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
Fix blackhole address
Add note about needing mailutils to read mail.
Clarify text.
Add package install
Use default alaveteli path.
More explanation of dc_use_split_config
Use example.com as example domain
Needs to be privileged