The default email server for Debian is exim. This can be set up in one of several ways, depending on how you will access your email. The exim configurator will list the options. The newer exim4 will become the default at some time and its configuration is somewhat simpler. To configure it:
$ wajig reconfigure exim4-config
The file /etc/exim4/update-exim4.conf.conf contains the simple configuration information which is used to generate the more complex exim4 configuration files.
The file /etc/email-addresses can be used to generate a From: address for each user (so that the From address will not be the default of the [email protected]).