First of all, it is against a number of RFCs to modify the ‘Subject:’ header of messages. However, it is frequently requested by users who have seen it on other list managers. Second, it is many times worse to have a prefix that changes from message to message, such as a prefix with the message number. However, a number of lists, especially in Japan, use this feature and in its absence these lists might be unable to take advantage of ezmlm. Thus, while we recommend against using a prefix, ezmlm-idx supports it.
To add a subject prefix, just put the text into DIR/prefix. The only format that makes any sense is ‘list:’ or ‘(list)’ or such.
The message number prefix is activated by putting e.g. ‘(list-#)’ into DIR/prefix. ‘#’ is replaced by the message number. ezmlm refuses to make more drastic changes in the subject of a message. As a consequence, the message number prefix is added only when the subject does not already contain a prefix. Thus, replies will have the message number of the original message. Doing anything else and still supporting rfc2047-encoded subjects in the archive threading (much more important) would require decoding the subject, removing/editing the prefix, and re-encoding the subject. This is far too invasive.
The entire thread can always be retrieved by sending a message to ‘list-thread-x’ where ‘x’ is the message number in the prefix of any message in the thread.