how to send HTML mail by this module

Enable Mime Mail? That will send HTML mail.

Or do you want the textareas to be WYSIWYGs?

I need this too. Installed htmlmail module. Works fine, the received mail message is in html format.

The textfield doesn't show the wysiwyg editor yet. For that I use a form alter, changing the field type from 'textarea' to 'text_format'.

But that throws an error. Caused by the fact that the form alter makes an array of $form_state['values']['message'] instead of a single value. So I added an additional validation handler to the form. Below the complete code.

function MYMODULE_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'node_registration_registrations_broadcast_form') {
    $form['message']['#type'] = 'text_format';
    $form['#validate'][] = 'MYMODULE_form_validator';
  }
}


function MYMODULE_form_validator(&$form, &$form_state) {
  $form_state['values']['message'] = $form_state['values']['message']['value'];
}

More correct would be

$form_state['values']['message'] = check_markup($form_state['values']['message']['value'], $form_state['values']['message']['format']);

because that's what the format is for: to run filters, and check_markup() does that.

That IS correct, thanks.