In order to enable that, you need to reconfigure exim (which is the MTA installed by default). Run dpkg-reconfigure exim4-config. Select the Internet mode and answer the few following questions about your domains (watch out for open relay issues). Exim will then restart.
You can test your configuration by running sendmail your-address. Type in the body of your message then type CTRL + D on the next blank line to send your message. Hopefully, it made it to your inbox.
To make sure everything is okay and your server has not become an open relay, have Online.Net test your mail server.