Deny spam with Exim4

["Casey T. Deccio" <casey@deccio.net>]

I am getting a lot of spam on my exim4 mailserver, much of which is
logged with "no host name found for IP address ..." and "no IP address
found for host ...".  The acl_check_rcpt acl is mostly set with the
debian defaults.

I enabled CHECK_RECPT_VERIFY_SENDER, which denies for "!verify =
sender".  AFAICT this has not denied any spam that has come in, so at
this point, I will probably disable it again.

I enabled CHECK_RCPT_REVERSE_DNS, which warns if the reverse host lookup
on the sender fails--and adds a X-Host-Lookup-Failed header.  This
seemed to mark much of the spam.  I considered modifying it to use
'deny', instead of 'warn', but I'm concerned about false positives.

Is there a better way to use CHECK_RCPT_REVERSE_DNS, either to deny at
the rcpt level, or to utilize the X-Host-Lookup-Failed header that is
placed on it?  I have used SpamAssassin, but I wasn't getting the
desired results before, so before I turn it on again, I would like to
try to filter out as much at the mail server as possible.

Thanks,
Casey