code: public function append_signature( $message, $signature = '' ) {
  
  		if ( empty( $signature ) ) {
  			return $message;
  		}
  
  		$separator = apply_filters( 'iac_signature_separator', str_repeat( PHP_EOL, 2 ) . '--' . PHP_EOL );
  
  		return $message . $separator . $signature;
  	}
docs: apply a signature-text to the email message
  
  @since 0.0.6 (2013.01.13)
  
  @param string $message ,
  @param string $signature (Optional)
  
  @return string