IP spoofing and spamming

Wed Oct 29 01:11:21 UTC 1997

Please no religionics.  Part of the below is true - part is what will happen
in the near future:

I have a spammer I am trying to block.  He is multihomed to me and ISP X.
He has address a.b.c.d from me and address a.b.c.e from ISP X.  Users
started seeing spams from a.b.c.e and complained to ISP X.  He shut off SMTP
to the customer but the spamming continued.  Turns out the user defaults out
to me no matter what, so his address was a.b.c.e when coming out of me.  For
me that is a spoofed address.  I then go to block his spoofed address.  User
then says, it is a valid address and I have no business blocking his IP
addresses, whether he has them from me or ISP X.  I then say I'll block SMTP
and the user says, "show me one letter from a user on the Internet
complaining to you that I am spamming".  Since his dns is located elsewhere
and since the IP addresses are not mine, the users aren't complaining to me
- but to ISP X and perhaps ISP Y (providing him secondary DNS service).  All
the ISP X & Y attempts to shut out the spam aren't affective due to the
multihoming.

What do we do in these cases?

Thanks,
Hank