Gotta download the email plugin for SMTP.
Fill it out with your info, or another client instead of local. (I use Amazon SES SMTP account details.)
Is there a setting or work-around to disable new sign ups?
You can disable local registration in the ACP. Relevant thread here: https://community.nodebb.org/topic/11810/how-do-you-disable-default-login-registration-and-enable-on-login-registration-through-sso-plugin