PHP’s FILTER_VALIDATE_EMAIL validates email addresses quickly and easily.

<?php
  $email = "marc@somehost.com";
  if(filter_var($email, FILTER_VALIDATE_EMAIL) === FALSE)
  {
    echo $email." is invalid";
  }
  else
  {
    echo $email." is valid";
  }
?>

Read it all at http://www.phpbuilder.com/columns/marc_plotz10012009.php3?page=2