function sendEmail() {
  var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  var currentsheet = spreadsheet.getSheetByName('Sheet1');

  var data = currentsheet.getDataRange().getValues();
  var html = HtmlService.createTemplateFromFile('TEMPLATE'); // create a html file, named template case sentitive

  var date = new Date();

  //loop over each line
  for(var i = 1; i < data.length; i += 1) {

    var status = data[i][1];

    if(status != 'sent') {

      html.message = data[i][0]; // Variable <?= message ?> on template

      var template = html.evaluate().getContent();

      MailApp.sendEmail({
        to: 'to_email@email.com',
        subject: 'Phrase here',
        htmlBody: template
      });

      currentsheet.getRange(i+1, 2).setValue('sent');
      currentsheet.getRange(i+1, 3).setValue(date);
      break;
    }
  }
}
function onOpen() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  ss.addMenu('Send email', [
    {name: 'Process emails', functionName: 'sendEmail'}
  ]);

}