Airwave uses postfix
For general basic configuration go here
http://www.postfix.org/BASIC_CONFIGURATION_README.html
As for Airwave, goto AMP setup -> General
You will see the following, choose yes to enable the mail server, then go create triggers
If you want to use a relay, you can set an IP address and test as well.
If you need to use something like a Gmail smtp server which requires authentication, then refer to the postfix help forums on how to configure that.