The pointers method is used by most commercial mail archival software that I've seen (the kind big companies deploy for SabOX compliance).
The problems with Pointers & de-dupe is that at least marketing emails are personalised, each are sent independently often with at least a minimum of personalisation, for example calling you by your name (people are more likely to read it if it calls them by name). You can do de-dupe, but your limited to the efficiency and overhead of your block size. Compression would also take the storage required down to ~ 1/10th (excluding attachments), but I don't think thats still enough. Even if deduping and pointers gave you a 500x storage saving, the data size for a few days email is still so unfathomable that it won't work as a single global system.
As far as trying to solve it with economics, its been tried (as a method to stop spam) but the question is who has to pay who and for what? Every such scheme has failed because you end up missing one of the big groups of users as it becomes un-economical for their mail pattern to continue. If you make the senders pay, few comercial entities want to play, and few people will provide free email. If you have to pay to store then how do you stop being billed for someone flooding you.