I am confused about the conditions  when server sends email notification .

I guess the following conditions :

1. Initially used disk 0 and user upload a file less then 1500 : No notification.
2. Then User upload another file and total disk usage cross/equal 1500  : Send email.
3. Now total disk usage =>1500 and user  upload another file : No notification.
4. User delete a file and disk usage<1500.
5. User again upload a file and total disk usage cross/equal 1500  : Send email.

Am I missing anything or any wrong condition  ?

Thanks