To enable Keycloak to send emails you need to provide Keycloak with your SMTP server settings. If you don't have a SMTP server you can use one of many hosted solutions (such as Sendgrid or smtp2go). To configure your SMTP server, open the Keycloak Admin Console, select your realm from the drop-down box in the top left corner. Then click on Email in the menu at the top. You are required to fill in the Host and Port for your SMTP server (the default port for SMTP is 25). You also have to specify the sender email address (From). The other options are optional. The screenshot below shows a simple example where the SMTP server doesn't use SSL or TLS and doesn't require authentication.

As emails are used for recovering usernames and passwords it's recommended to use SSL or TLS, especially if the SMTP server is on an external network. To enable SSL click on Enable SSL or to enable TLS click on Enable TLS. You will most likely also need to change the Port (the default port for SSL/TLS is 465).

If your SMTP server requires authentication click on Enable Authentication and insert the Username and Password.