We discussed this, at my urging, on 2005-08-18 to 2005-08-23 on LSTOWN-L. 

Basically, a dropspam.com agent in your subscribers computer "calls 
home" and dropspam sends the notification to you, as you say, without 
any inkling of who it is. 

In my case, a letter to dropspam.com via their "Contact Us" web form was 
promptly responded to.  My letter was harshly worded by a lawyer, and 
eliminated the problem with my subscriber.  Since I haven't heard of 
other dropspam problems, I wonder if they made their software smarter or 
white-listed my server? I don't know.

Yes, I think your suggestion in the last line is the only (other) way to 
resolve this ...  unless you can buy them out and then drop the service! ;-)

cheers, wayne

Listserv Manager wrote, in part,  on 10/7/2005 4:16 PM:
> I've sent this to support, but since I'm trying to deal with this quickly,
> I'm also asking the user community.
>
> One of our lists with over 6k subscribers, has a potentially
> difficult situation that I believe could affect any listserv list. One of
> their members subscribed to drop_spam. When a post went to the list, the
> outgoing post to the dropspam subscriber was intercepted by drop_spam.
> Drop_spam then spoofed and email to the original sender, appearing to come
> from lm_net, telling the sender to click on a link to approve the message.
> Clicking on the link is to register with dropspam. This obviously causes
> mass confusion, and gets dropspam a bunch of registrations under false
> pretenses. The list has been notified NOT to click on the link, but a
> number probably already have. We would like to identify folks who have
> registered with dropspam so we can deal with this. My first thought is to
> do an immediate active probe which would generate, but dropspam will
> probably sanitize their email of any indication of the sender. Any
> suggestions? A custom email to all subscribers with a unique
> subject?