Umbraco-CMS/src/Umbraco.Web.UI.NetCore/appsettings.Development.json

{
    "Umbraco": {
        "CMS": {
            "Global":{
                "Smtp": {
                    "From": "your@email.here",
                    "Host": "localhost",
                    "Port": "25"
                }
            },
            "Hosting":{
                "Debug": true
            }
        }
    }
}