If you enable the Changelog feature for your list:

* Change-Log = Yes

you'll create a log file which records all posts and subscription changes
for your list.  This won't do you any good for old data (the log will
only start recording once you enable the feature), but it will provide you
with the data you want in the future.  A sample:

20011128111933 SET [log in to unmask] NOREVIEW
20011126135749 POST [log in to unmask] Emergency Virus Alert

The list owner can retrieve the changelog file with a GET command:

GET LISTNAME.CHANGELOG

Hope this helps.

--
Liam