Sometime Today, A cobbled together some glyphs to say:
Currently we're on Mailman 2.1.5-8 from debian stable. These default messages are changable through source only. I could do that, but we'd have to patch mailman everytime we upgrade it.
The correct thing to do would be to submit the patch to the mailman maintainers.
A better way may be to make these messages configurable via a config file on the server (could just be a python include file that defines some constants).