Something

<script type="text/javascript">

function mail_select(){

var checkboxes = document.getElementById('checker');
alert(checkboxes[0].value);
	if(checkboxes <= 0){
		
		document.select_form.email_option.selectedIndex = 0;
	
		return false;
	
	}

}

</script>

<form name="select_form">
  <select onchange="mail_select()" id="select_mail" name="email_option">
		  <option class="select_1" value="0">Flere muligheder...</option>
		  <option class="select_2" value="1">&nbsp;&nbsp;&nbsp;&nbsp;Markér ulæst</option>
		  <option class="select_2" value="2">&nbsp;&nbsp;&nbsp;&nbsp;Markér læst</option>
		  <option class="select_2" value="2">-----------</option>
		  <option class="select_1" value="0">Flyt til mappe</option>
  </select>
</form>

<form name="checkbox_form">
<input class='mail_over' type=\"checkbox\" name=\"checker[]\" id=\"checker\" value=\"$r[id]\" />
<input class='mail_over' type=\"checkbox\" name=\"checker[]\" id=\"checker\" value=\"$r[id]\" />
<input class='mail_over' type=\"checkbox\" name=\"checker[]\" id=\"checker\" value=\"$r[id]\" />
</form>