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?
[Default]
Mail name?
Your mailserver's primary hostname. (Fully qualified)

# mkdir /etc/postfix/virtual

#touch /etc/postfix/virtual/domains

example.com
example.net
example.org

#touch /etc/postfix/virtual/addresses

example.com              DOMAIN
bob@example.com          bob
steve@example.com        steve

example.net              DOMAIN
@example.net             steve

example.org              DOMAIN
@example.org             chris

References:

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