Most solutions for search presuppose an open list hosted on the Internet that can be accessed by search engines like Google. This is obviously not going to work for an internal corporate list that needs to be kept confidential.
One solution for internal lists might be to integrate mailman with another open source product, called Swish-e.
No, that’s “Swish”, not “Swiss”. This has nothing to do with CERN or anything else in Central Europe.
There is a how-to on Integrating Mailman with a Swish e-search engine.
In addition to the aforementioned article, you may also want to look at Listserv to Mailman Part 1.2: Installing Swish for Archive Searching, which does a good job of giving practical advice on how to address the major architectural issues. There’s a follow on article here on how to actually set up searching.