Mozilla Mailnews FAQ

How do I purge newsgroups of old messages?

Depending on what you want to purge, there are a couple of ways to go about it. If you want to get rid of messages that have expired from the server, select the earliest message in that newsgroup. If that article has expired, you should see a message in preview pane telling you that the article has expired from the server, and it will have a link you can click on to "Remove expired messages".

Note: When you click on "Remove expired messages", a second mailnews window will open. It's a bug in Mozilla. Close the second window, when the purge is finished. No harm will be done.

If you want to remove messages that may still be available on the server, you'll have to close Mozilla, and delete the newsgroup MSF file. (see: Transferring Newsgroups) Deleting the MSF file removes all headers for that newsgroup; but don't worry about remembering which messages are marked as read. That info is kept in another file. The reasoning behind deleting the MSF file is that you will asked to download headers the next time you access that group. You can choose the number of messages (headers only) downloaded. The caveat in deleting the MSF file is that you will loose message markings. Messages/threads previously marked watched, flagged, labelled, or replied/forwarded will no longer have those markings.