With all due respect Jeff, I am making a request for a feature. I have no desire to write one as I am not in the business of writing plugins.
I'm sorry if you took my response personally; I was merely trying to point out the problems inherrent in this kind of solution. Why? Because the way to implement your request is in exim.conf. And currently I'm maintaining DirectAdmin's exim.conf file.
Please feel free to ignore the rest of this response; it simply explains why challenge-response systems are a bad idea.
It is the choice of the client to use it. Many have requested it and generally understand how it works.
I'm not so sure. For example I don't think you understood what I mean by a form response and why it creates a problem.
For an example, look at my home page,
here; you'll see a
Subscribe to our Newsletter link near the bottom left. That's a form. It's processed by a form-to-email program and the responses come to us in emails. That's how most website owners get responses to forms on their websites.
Normally the workflow is simple; the form (in our case a perl script but it could just as easily be a php form) sends us an email from the email address the visitor filled in. Then we can subscribe (manually or automatically) the visitor to receive our emails. (In fact, we write them first to make sure they want to subscribe, and that someone else didn't fill in their information maliciously.)
However, if we were using BoxTrapper the workflow would be a bit more complex:
The email would come to the address we have set up for it. It would be intercepted by BoxTrapper. BoxTrapper would tell them that they've written an email to us, and that we won't accept it until they approve that it was really sent by them. Since they know they didn't send us an email they'd be confused and it's doubtful they'd approve their, as far as they know, unsent email.
Since it's only a newsletter request, who cares. But what if it's more than a newsletter request. What if it's an enquiry for an expensive product?
It's my feeling that customers won't understand that they'll no longer get most inquiries directed to their site if they run something like BoxTrapper, and that they'll blame the hosting company for losing their email.
And even if they do understand the problem, it's your server that's, sooner or later, going to end up sending spam. And this is why I suggested you read about challenge-response solutions.
I presume you don't want your server to be an open relay, because then spammers would use it.
BoxTrapper and other challenge-response systems have a similar problem.
Spammers send out emails with forged from addresses. If one of your addresses ever gets forged as a from address by a spammer you'll see the problem. Lots of servers using challenge-response will send challenges to you. Sometimes hundreds of thousands of them.
Now consider what happens when a spammer forges some innocent person's email address as the from address on a dictionary-attack spam to a site on your server. A site using a catchall email box. Say they try 1000 emails to nonexistent addresses at the domain hosted on your server. At this point your challenge-response system is going to send 1000 challenges back to that innocent person.
Which would make you a spammer. Which would get your server put on one or more blocklists; possibly the SORBS blocklist; they charge to remove you. Which might even get your upstream provider mad at you.
Perhaps DirectAdmin staff will see your post and create a feature such as BoxTrapper.
Perhaps some third party company will look at your post, and decide to create it as a plug-in or an alternative exim.conf file.
I'd still recommend you don't use it.
Jeff