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