Sendmail is an open source smtp server. Many different languages and frameworks interface with sendmail, and when debugging it's good to be able to quickly test whether or not sendmail is working.

The following is a quick way to verify that sendmail is working.

echo "Subject: test" | /usr/lib/sendmail -v me@domain.com
Confirm your edition with a simple command.
Ash Ash (343)
2 minutes

So you've got a machine running Ubuntu. If it's been a while since you set up Ubuntu (or maybe you're just curious), you probably need to check what version you're running.