Now this is a bit tricky, but works great if you can decide which Top Level Domains or TLDs you want to receive mail We are getting so much spam from countries we never receive useful mail, I've been experimenting with blocking entire TLDs using sendmail access_db as an antispam technique.
Here is my current list, working great... yes I might miss a good one, but it blocks SO MUCH junk 
:
ad REJECT
ae REJECT
af REJECT
al REJECT
ar REJECT
ao REJECT
az REJECT
ba REJECT
bb REJECT
bd REJECT
bg REJECT
bi REJECT
bh REJECT
bj REJECT
bt REJECT
bw REJECT
by REJECT
cf REJECT
cg REJECT
ch REJECT
ci REJECT
cm REJECT
cn REJECT
cs REJECT
cy REJECT
cz REJECT
dz REJECT
ee REJECT
eg REJECT
eh REJECT
er REJECT
et REJECT
ga REJECT
ge REJECT
gf REJECT
gh REJECT
gm REJECT
gn REJECT
gp REJECT
gq REJECT
gw REJECT
gy REJECT
hr REJECT
ht REJECT
hu REJECT
id REJECT
iq REJECT
ir REJECT
jm REJECT
jo REJECT
ke REJECT
kg REJECT
ki REJECT
kp REJECT
kr REJECT
kw REJECT
lb REJECT
li REJECT
lr REJECT
ls REJECT
lt REJECT
lv REJECT
ly REJECT
ma REJECT
md REJECT
mg REJECT
mk REJECT
mn REJECT
mr REJECT
ms REJECT
mt REJECT
mu REJECT
mv REJECT
mw REJECT
mx REJECT
my REJECT
mz REJECT
na REJECT
ne REJECT
ng REJECT
ni REJECT
om REJECT
pa REJECT
pe REJECT
pf REJECT
pg REJECT
pk REJECT
pl REJECT
pn REJECT
pr REJECT
pt REJECT
py REJECT
qa REJECT
ro REJECT
ru REJECT
rw REJECT
sa REJECT
sb REJECT
sc REJECT
sd REJECT
si REJECT
sk REJECT
sn REJECT
so REJECT
sr REJECT
su REJECT
sv REJECT
sy REJECT
sz REJECT
td REJECT
tg REJECT
tj REJECT
tk REJECT
tm REJECT
tn REJECT
to REJECT
tp REJECT
tr REJECT
tt REJECT
tu REJECT
tv REJECT
tw REJECT
tz REJECT
ug REJECT
uy REJECT
uz REJECT
ve REJECT
ws REJECT
ye REJECT
yu REJECT
za REJECT
zm REJECT
zr REJECT
zw REJECT
Combining this TLD blocking with Procmail charset filters has cut my spam down to fraction of what it was.......