public function setField( Net_Mail_Header_Field $field, $emptyBefore = TRUE )
	{
		$name	= strtolower( $field->getName() );
		if( $emptyBefore || !array_key_exists( $name, $this->fields ) )
			$this->fields[$name]	= array();
		$this->fields[$name][]	= $field;
	}