Though the minimum polling frequency is 1 minute, setting the polling frequency to less than 15 minutes may not be applicable for all the monitors including Mail Delivery Monitor.
This is because every monitoring capability goes through different mechanisms for data collection, which requires more time than the usual monitoring time and may create overhead to the server. Hence the minimum polling frequency is 15 minutes for monitors like Mail Delivery Monitor, Web Page Speed (Browser) etc., which can't be altered.