[exim] re using more diskspace

hi,
if we require more disk space on exim and that much diskspace is not
present on a single server then how can we expand on diskspace, we can
definately mount another server space by nfs but how can we use that
for email storage, whar does yahoo and gmail do for this. is this
achievable by simply changing the config file or some code
modification is required in exim.

plzz help
Deepak