You should check out Lurker as a 3rd-party archiver. Integration of Lurker with Mailman could be somewhat improved (especially the UI), but it works quite well already.

Generally, I don't think Mailman should try to create its very own archiver. It's just not Mailman's core competency.

MailmanWiki: DEV/ModernArchiving/0002 (last edited 2007-07-31 19:41:14 by julian@mehnle)