I've noticed a HUGE number of people have email addresses on other diarists notify lists which are no longer valid and are bouncing. The mailbox for the user who runs the webserver process (and is the sender of all notify mails) was over 160Meg when I returned to the UK!!
To that end, I have installed a script that will REMOVE all notifications from addresses that bounce in excess of three times. For most of the bounces this won't be a problem, but sometimes mail server glitches may mean you inadvertantly get unsubscribed. If this happens, you'll need to resubscribe to your favourite diaries again.
So far, and the script has only been in place an hour, there are 104 email addresses with a permanent, fatal, bounce!
Any other messages to the user who runs the web server are ignored and just silently deleted. So don't try to write to it :-)
Steve.