Hate to get off topic here but, I have to disagree with you on quotas. Quotas are a good thing(tm). "Time Burden" being key here; anything the user can do, should be done by the user. Unless you have a bazillion admins (never seen that before) then the user needs to accept some responsibility for what I consider to be trivially easy.
I really don't care who you are, from peon to CEO, part of your job is proper e-mail archival. Keep things nice and tidy with filters. Set automated pruning according to your requirements. Every so often, delete some stuff by hand, or burn it to a CD.
It's just like any other type of filing. If you were an accountant would you leave a huge pile of bills and invoices on your desk until it was so big you couldn't get to your desk?
/* Halley */ (Halley's comment.)