MS Anti Spam Question.

[Wiley Coyote]

When I block an email address all works fine, however when I want to block an
entire domain, email sneaks thru. Here is an example:

Email to Block: (e-mail address removed) This works fine,
Domain to Block: @bob.com Some emails sneak through, but mostly blocked,
Domain to Block: @ab.bob.com Same as above (this is a sub domain, so ???)

Now I've checked the email headers, addresses etc and all seems to be fine (i.e.
the addresses do not appear to be spoofed). I did dns lookups, lpad queries etc
and as stated all seems fine.

Does anyone have any ideas as to why some DOMAINS are blocked just fine, while
others aren't?

Thanks...
ps: NOTE the above email addresses and domains are fictional.