Sending email from Ubuntu instance

I created a Linode instance using the marketplace "Secure Your Server" app/guide. There was a question in there about "do you want to send email from this domain". I wasn't sure what that did, so I said "no".

First, what does this do? Second, is this something I can change after the fact? I want to be able to send admin email notifications from my server. Do I need to install an email server from the marketplace to do this?