To install postfix run, as root:

# apt-get install postfix

Once installed you'll be presented with a simple debconf setup wizard. Proceed by answering the questions as follows:

General Type of Configuration?
Internet Site
Where should mail for root go?
Mail name?
Your mailserver's primary hostname. (Fully qualified)

# mkdir /etc/postfix/virtual

#touch /etc/postfix/virtual/domains

#touch /etc/postfix/virtual/addresses              DOMAIN          bob        steve              DOMAIN             steve              DOMAIN             chris


Handling mail for multiple virtual domains with postfix
Tips for a Debian GNU/Linux System Administrator.