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If you like to reenable archiving later, just turn on archiving in the list-administration panel (and set it to public if you like to).
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NOTE ON PRUNING OLD MESSAGES:
There is a script at http://www.msapiro.net/scripts/prune_arch
(mirrored at http://fog.ccsf.cc.ca.us/~msapiro/scripts/prune_arch) which can prune messages older than a specified number of days from the archives of one or more lists.
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HOW TO DELETE A MESSAGE FROM THE HTML ARCHIVE ONLY
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