However I do think there are perfectly legit emails, particularly in a large organisation, that have a very limited shelf-life and qualify for both should-be-sent and should-be-auto-trashed.

Examples - "Are you still heading up that project?" "Hey, I have this tech issue, can you help?". In both of these cases a quick reply would be useful, but in a week's time it's likely the issue has gone away or the person is now so stuck they'll remind you about it.

I guess you could argue that email is not the appropriate comms method for those questions...