idled vs. notifyd ?

[Patrick Goetz]

In my previous debian-based cyrus install idled was commented out in 
cyrus.conf, while the notifyd service was set to run.  Not knowing any 
better, I just left it like this.

Now that I'm going over the entire configuration in great detail, I find 
out that idled would be pretty useful thing to have running (since we 
use mostly the Thunderbird MUA, which supports IDLE), while I'm not even 
sure what notifyd does.

Any clues?