PHP if..else option in an else section

I'm not sure where the rest of the code is....but it would be something like this:
else if($_POST['txtUsername']=='') {
    echo "Invalid email address.";
}

Hi
Thanks for that..but,,
when the page first opens the message still comes up with INVALID EMAIL which is what i am trying to get rid of..
I just need it to come once the user has pressed LOGIN - which i need to change to SUBMIT which i have just done..
cheers
mat

<p>Please register your email to gain access to vocal stems.</p>
<form name="form" method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>"> 
    <p><label for="txtUsername">Email Address:</label> 
    <br /><input type="text" title="Enter your Email Address" name="txtUsername" /></p> 

    <p><input type="submit" name="Submit" value="Login" /></p> 

</form> 

<p>
  <?php 

if(eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $_POST['txtUsername'])) { 
  { 

?> 
  
  Right Click on link to download the file</p>
<p> <a href="xxxxx">b vox 1</a></p>
<p>
  <a href="/xxxxxx">b vox 2</a></p>
<p>
  <a href="/xxxxxx">lead vox</a>

<?php
 $to = "XXXXXXXXXXXX";
 $subject = "REMIX COMPETITION - SOMEONE HAS DOWNLOADED THE STEMS!";
 $body = "Another entry in the REMIX Competition from - {$_POST['txtUsername']}";
 if (mail($to, $subject, $body)) {
   echo("<p></p>");
  } else {
   echo("<p>.</p>");
  }
?>

<?php

$myFile = "remixcomp/mercyremix.txt";
$fh = fopen($myFile, 'a') or die("can't open file");
$stringData = "{$_POST['txtUsername']} \n";
fwrite($fh, $stringData);
fclose($fh);



?>
  <?php 

} 

} 
else if($_POST['txtUsername']=='') { 
    echo "Invalid email address."; 
}

?>

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Genius!
Worked like a charm!
Thanks very much! :-)

Genius!