What email should I change it for?
The one where you want to receive all the emails. For example : You want any user interested to become an instructor in your site to email you his details (name, qualification, university etc..) at some specific email address, then you can specify that email address here. Once you get the email and feel that this candidate is suitable, you can create a user for her from WP ADmin - Users - Add New section and then that user can start creating courses in your site.
Ok, Are you getting the same error : "Unable to send email", Then the issue could be in your server. Try contacting your web host for this.
You can try the WP Mandrill plugin (Free) by MailChimp.
a. Go this this site
Mandrillapp.com
b. Create an account.
c. Generate license keys in setting.
d. Install this plugin https://wordpress.org/plugins/wpmandrill/
f. Add the license key in WP Admin - Setting - Mandrill
g. Now any email you send through your site will be processed by Mailchimp SMTP mail servers instead of your web host servers.