We have two providers available `strapi-email-sendgrid` and `strapi-upload-mailgun`, use the alpha tag to install one of them. Then, visit `/admin/plugins/email/configurations/development` on your web browser and configure the provider.
If you want to create your own, make sure the name starts with `strapi-email-` (duplicating an existing one will be easier to create), modify the `auth` config object and customize the `send` functions.