I don't know how much flexibility you have in making changes to the MX servers, but personally, I would take one of the following approaches:

1. Preferred approach: have the MX server open an SMTP connection to the listserv server, check the address with a RCPT TO and see what happens. This does cost a bit more than an LDAP query, but has the advantage of complete black-box compatibility. You can change the LISTSERV server at will and this method will still work. I also think such a mod would be generally useful and likely to be adopted by the vendor.

2. Alternate approach: what you really need is two things. One, a way to find out if a given list exists or not. This could be an LDAP entry (one per list) or a flat text file or whatever other method makes the most sense given the software you are using. Two, a few lines of code that, given a list name, match any of the associated mailboxes.

But I really prefer #1 because it is generic and can be used for all kinds of things beyond LISTSERV lists.

  Eric