The British National Corpus (see <http://info.ox.ac.uk/bnc>) contains a
fair amount (I don't know exactly how much) of business and private
correspondence. However, although the BNC is supposed to span around 20
years, and does for published works, the unpublished material is mostly
from the late 80's and early 90's, so you won't get that time span you
want. May be worth a look though.
Note for other readers: the BNC is currently only available to researchers
within the European Union. There is a good chance that this will change
soon.
-- Dominic Dunlop