Sendmail not working on Craft 3
Most servers (speaking in generalities) won't send mail by themselves anymore. You'll need to use some sort of SMTP service. 

Check out SendGrid to get you going. I believe they have a free plan. You can choose SMTP from the mail options from the settings page.

Choose 'Use Authentication' and from there your username is well, your username and your password is...your password. Use Port 587.

Hope this helps!