var mail = {
    properties: {
        __metadata: { 'type': 'SP.Utilities.EmailProperties' },
        From: 'from@mail.com',
        To: { 'results': ['one@mail.com','two@mail.com'] },
        Body: 'some body',
        Subject: 'subject'
    }
};

var getAppWebUrlUrl = decodeURIComponent(utils.getQueryStringParameter("SPAppWebUrl").replace("#", ""));
var urlTemplate = getAppWebUrlUrl + "/_api/SP.Utilities.Utility.SendEmail";
$.ajax({
        contentType: 'application/json',
        url: urlTemplate,
        type: "POST",
        data: JSON.stringify(mail),
        headers: {
            "Accept": "application/json;odata=verbose",
            "content-type": "application/json;odata=verbose",
            "X-RequestDigest": $("#__REQUESTDIGEST").val()
        },
        success: function (data) {

            // code

        },

        error: function (err) {

            // code

        }
    });