Little helped needed on feedback form

ok well ive made a feedback form for a company, it all works however all i wont to do now is add like a if statement, so if the email box hasn’t had any information added in it will echo a error saying please fill email email box.

here is my php code


 <?php
if(isset($_POST['submit'])) {

$to = "[email protected]";
$subject = "Bentek Feedback Form";
$name_field = $_POST['name'];
$email_field = $_POST['email'];
$message = $_POST['message'];
$company_field = $_POST['company'];
$telephone_field = $_POST['telephone'];
$position_field = $_POST['position'];
 
foreach($_POST['check']  as  $value)  {

$check_msg .= "Checked: $value
";

}

$body = "

From: $name_field

E-Mail: $email_field

Company name: $company_field
 
Telephone No: $telephone_field
 
Position in Company: $position_field
 
$check_msg
 
Message:
 $message";
 
echo "Data has been submitted to $to!";
mail($to, $subject, $body);

} else {

echo "There was a Error !";

}
?>
<FORM action=mailer.php method=post>
<table width="359" height="490" border="1" align="center" cellpadding="0" cellspacing="0" bordercolor="#000000">
  <tr>
    <td width="355" height="484" align="center" valign="top"><table width="355" border="0" cellpadding="0" cellspacing="0">
      
      <tr>
        <td height="26" colspan="2" align="center"><div id="headerco">Feedback Form</div></td>
      </tr>
      <tr>
        <td height="9" colspan="2" align="center">&nbsp;</td>
      </tr>
      <tr>
        <td width="186" height="31" align="left"><div id="name">&nbsp;&nbsp;Name:</div></td>
        <td width="169"><INPUT size=19 name=name></td>
      </tr>
      <tr>
        <td height="29" align="left"><div id="telephone">&nbsp;&nbsp;Telephone No:</div></td>
        <td><INPUT size=19 name=telephone></td>
      </tr>
      <tr>
        <td height="28" align="left"><div id="email">&nbsp;&nbsp;Email:</div></td>
        <td><INPUT size=19 name=email></td>
      </tr>
      <tr>
        <td height="29" align="left"><div id="company">&nbsp;&nbsp;Company Name:</div></td>
        <td><INPUT size=19 name=company></td>
      </tr>
      <tr>
        <td height="28" align="left"><div id="posititon">&nbsp;&nbsp;Position in Company:</div></td>
        <td><INPUT size=19 name=position></td>
      </tr>
      <tr>
        <td height="23" valign="top">
          <p>            </p>          </td>
        <td height="23" valign="top"></td>
      </tr>
      <tr>
        <td height="83" align="left" valign="top" class="one">&nbsp;&nbsp;<INPUT type=checkbox value=computers name=check[]> 
           <span class="one">Computers</span><BR>
  &nbsp;&nbsp;<INPUT type=checkbox value=dictation name=check[]> 
            <span class="one">Dictation</span><BR>
  &nbsp;&nbsp;<INPUT type=checkbox value=network name=check[]> 
            <span class="one">Network 
            Installations</span><BR>
  &nbsp;&nbsp;<INPUT type=checkbox value=internet name=check[]> 
            Internet 
            Solutions<br />
            &nbsp;&nbsp;<INPUT type=checkbox value=consultancy name=check[]>
            Consultancy<BR>
            &nbsp;&nbsp;<INPUT type=checkbox value=furniture name=check[]>
          Furniture</td>
        <td height="83" align="left" valign="top" class="one"><INPUT type=checkbox value=copiers/printers name=check[]>
        Copiers/Printers
        <BR>
<INPUT type=checkbox value=hardware name=check[]> Hardware 
Support<BR>
<INPUT type=checkbox value=fax name=check[]> Fax 
Machines<BR><INPUT type=checkbox value=supplies name=check[]> 
Stationery/Supplies<BR><INPUT type=checkbox value=training name=check[]> 
Training<BR><INPUT type=checkbox value=software name=check[]> Software 
Support</td>
      </tr>
    </table>
    <br />
    <textarea name=message rows=5 cols=30>Message goes here !</textarea><br />
    <INPUT type=submit value=Submit name=submit></td>
  </tr>
</table></FORM>