A ezmlm thread is just a message number-ordered set of messages with identical “normalized” subject entries. This is a very reliable method for threading messages. It does not rely on any variably present ‘In-Reply-To:’ or ‘References:’ headers. If the subject changes, the continuation becomes a separate thread very close to the original thread in a digest. ezmlm-idx uses this mechanism to return message sets threaded and with a thread and author index, unless specifically told not to do so with the ‘n’ format specifier. Naturally, lists set up without a message index (using the ezmlm-make -I switch) do not maintain thread information.
The ezmlm-archive(1) can be used at any time to either reconstruct an existing thread index or create one a thread index for an existing message archive without one. Use the ‘-c’ option to rebuild the indices from scratch.