I was looking at the email/messaging example and noticed that a lot of immutable data is left behind for no real reason and also a potential security concern.
I propose having the option for the creator of immutable data to set the number of reads before the network automatically deletes the data.
So for private messaging, you could set immutable_reads = 1.
Each read of the immutable data would decrement this counter until it reaches 0 and is deleted.
A safe write as a self-deleting immutable data would probably cost the user less safecoin.