[exim] Not rejecting mail forwarded from friendly hosts

It is a good idea to reject likely to certain spam, because in the rare cases
of false positives the legitimate sender will be notified. It is also a good
idea to have at least as good spam protection at the (forwarding) secondary
MXes as at the primary. But what is the best way(s) to handle .forward-ed
mail coming from friendly but slightly stupid (in the sense that they lack
adequate spam protection) hosts, and how do you implement it with Exim?

a) Don't care; it's their problem if they can't identify spam as well as we
can! Or, in other words: We shouldn't do anything, because it will cost us
money and give us nothing but more spam in return.

b) Whitelist well-known providers (which should have adequate spam filters but
there is always some mail that will pass through one filter but not the
outher).

c) Monitor mail logs to identify forwardings automatically or manually.

d) User-managed ~/.backward (or a database or whatever) containing addresses
and/or hosts forwarded from.

Disscuss, please!

--
Magnus Holmgren