<?php
 
/**
 * Override Zend_Validate_EmailAddress so that only one error message shows when there is a problem
 */
class RPK_Validate_EmailAddress extends Zend_Validate_EmailAddress
{
 
  /**
   * Returns true if and only if $value is a valid email address
   * according to RFC2822
   * @param string $value
   * @return boolean
   */
  public function isValid($value)
  {
    $response = parent::isValid($value);
    if (!$response)
    {
      $this->_messages =
        array(self::INVALID => "Please enter a valid email address");
    }
    return $response;
  }
}

<?php
//in form object
$element->addValidator(new RPK_Validate_EmailAddress());