I signed up to check this out for you guys.
InboxResponse must keep record of the default responses from a huge range of mail server software. With many languages and customized responses this seems an impossible undertaking to me.
I noticed they are using a header addition (let me know if you need help adding that, and I would consider an option you might enable in the config file). When ListMail was created I used headers for bounce processing. I wanted to resend the -same- message that bounced, holding the user in sequence until they were recovered. It wasn't meant to be. Around 40% of servers did not return the headers when they bounced a message, making reliable tracking impossible. This is exactly why the bounce feature was changed to "X times in X days" with all processing being based on the email address itself.
Back to my first point... InboxResponse must either "scan for keywords" or track tens of thousands of bounce responses in order to determine the exact reasons for bouncing (every server has a different way of saying "Server said: 550 user unknown".
This gets me thinking. I could do more research in this area to improve ListMail's bounce tracking and might even be able to achieve some level of success by analyzing the various common email servers (postfix, qmail, sendmail, exim) and scanning their bounces for the numeric error codes. This could result in tracking such as seen on InboxResponse.com. An improvement like this would eliminate the need for a 3rd party such as InboxResponse.com to access your bounces - it would all be handled by ListMail running on your own site.
The ListMail bounce process is being improved for future updates to ignore certain bounces (AOL / Yahoo deferrals, HotMail connection issues, spamblocks, etc - it will be configurable) and again insert a tracking header for those servers that support it. I also plan to improve and include the AOL complaint removal script with ListMail along with instructions.
Regards