You have allowed your computer to act as an email server, however depending on your configuration the scope is most likely limited by the configuration of Sendmail and any firewalls. Most likely you will be able to send email from the local machine (and possibly IP's on your LAN), but without setting up things like an MX record your server will not receive email from the wider Internet.
Hopefully Sendmail is set up as a mail relay - ie to relay mail to another mail server which is set up as a "proper" mail server, otherwise you will have great difficulty with reliably using Sendmail to send email (because of Spam issues). I note that even if it is set up as a mail relay, technicaly it is still acting as a mail server.