Spam fighting using access.db 1

[DownWithSpam]

I have been having a lot of success reducing the volume of
mail that has to be content scanned by using the access.db
mechanism to block certain domains. An example would be to
block everything ending in dynamicip.<some domain name>.
Other key words of interest are ones like pppoe, dhcp, or
residential, which appear in the reverse lookup on ips which
are sending mail. I do not profess that this is always a good
idea, and it indeed needs to be used carefully, but it can
make a significant impact on content filtering requirements.

The problem is, and please correct me if I am wrong, the
access.db processing does not allow for wildcards in the middle
of the pattern, a match is only made when the access.db entry
exactly matches the end of the reverse lookup. IP blocks are
treated the opposite, but that's not at issue.

What I am looking for is advice, or even an example, of how
and where it would be appropriate to add rules that could handle
the case where the reverse lookup is of the form:
dhcp.<anything>.<some domain name>. The goal is to return a
550 error message based on the reverse lookup string and not
have to receive the message and content filter it.

Any advice, example, pointer to where this might be covered
already (I have done some searching but not found anything),
would be appreciated.