[Cialug] Sendmail spam rejection

[Daniel A. Ramaley]

I have what is hopefully an easy question for the sendmail wizards on 
this list. I have 2 mail servers. The first receives mail from the 
outside world, filters out most of the spam, and then passes the mail 
to the second server (which is what users actually log in to to check 
their mail). The first server runs sendmail. It does not currently have 
a way to verify the recipient addresses, so it just accepts all mail 
for the domain. Then when the first server forwards mail on to the 
second server, any invalid recipients cause a bounce to be generated.

I would prefer for the first server to never accept invalid recipients 
in the first place, before running expensive spam checks and clogging 
up the second server with bounces that usually never go anywhere. I 
have a nightly data feed with a list of valid recipients. What is the 
best way to load that list on to the first mail server and make 
sendmail check it as soon as it has received a recipient address?


So far Google has not been quite as helpful as i would have liked 
(probably because i'm not familiar enough with sendmail to search the 
correct terms), but i've found 2 possibilities. I don't know whether 
they are plausible, though. One would be to set up a virtusertable hash 
file with a list of valid recipients (all 20 000 of them), but i think 
that is mostly used to rewrite addresses. I don't want to rewrite 
addresses, i just want to have a list of valid ones and deny the rest:
    user1 at example.com    user1 at example.com
    user2 at example.com    user2 at example.com
    ...
    @example.com         error:"550 We do not accept spam"

Would that work? If not, the other idea i've seen would be to set up 
sendmail's ldap_routing feature, but instead of using real LDAP, use a 
hash table. Unfortunately, i've not yet found understandable 
instructions on how to use the ldap_routing with a hash file.

------------------------------------------------------------------------
Dan Ramaley                            Dial Center 118, Drake University
Network Programmer/Analyst             2407 Carpenter Ave
+1 515 271-4540                        Des Moines IA 50311 USA