private void SendEmail( ClientContext clientContext )
    {
        User sendToUser = clientContext.Web.EnsureUser( "my.mail@mail.com" );
        clientContext.Load(sendToUser);
        clientContext.ExecuteQuery();

        string email = Microsoft.SharePoint.Client.Utilities.Utility.GetCurrentUserEmailAddresses( clientContext ).Value;

        Microsoft.SharePoint.Client.Utilities.EmailProperties properties = new Microsoft.SharePoint.Client.Utilities.EmailProperties();
        properties.To = new string[] { sendToUser.Email };
        properties.Subject = "subject";
        properties.Body = "body";

        Microsoft.SharePoint.Client.Utilities.Utility.SendEmail( clientContext, properties );

        clientContext.ExecuteQuery();
    }

MailMessage mail = new MailMessage("from@mail.com", "to@mail.com");
SmtpClient client = new SmtpClient();
client.Port = 25;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Host = "smtp.google.com";
mail.Subject = "this is a test email.";
mail.Body = "this is my test email body";
client.Send(mail);

var spContext = SharePointContextProvider.Current.GetSharePointContext(Context);
using (var clientContext = spContext.CreateUserClientContextForSPHost())
{
    var emailp = new EmailProperties();
    emailp.BCC = new List<string>{"a@mail.com"};
    emailp.To = new List<string>{"b@mail.com"};
    emailp= "from@mail.com";
    emailp.Body = "<b>html</b>";
    emailp.Subject = "subject";
    Utility.SendEmail(_clientContext, properties);
    _clientContext.ExecuteQuery();
}

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

            }
        });