Note that "can't access mailbox" is mentioned distinctly from "no such mailbox".
You're reading too much into that. "Can't access mailbox" covers such normal cases as protection failures.
It is not a mandate that all instances of "horrible error #69" must be handled in that way.
"Horrible error #69" conditions shouldn't ever happen. Rather than discuss the right way to present them to the user you would be much better off preventing them from happening in the first place. I personally feel that trying to be clever about handling an "impossible" error interferes with the diagnosis and repair of the problem.
-- Mark --
http://staff.washington.edu/mrc Science does not emerge from voting, party politics, or public debate. Si vis pacem, para bellum.
