Sometimes people want to use Mailman's built-in pipermail archiver and also invoke some process such as an indexer for a search engine or another archiver on messages as they are archived.
This normally cannot be done by a handler in the processing pipeline because processing and archiving are done asynchronously by different processes, and a handler in the processing pipeline has no way to reliably know the ultimate location of the message in the archive.
Mailman does have a facility for invoking an external archiver to archive the message, but this facility bypasses the built-in pipermail archiver. Thus there is a need for an external archiver that can invoke the pipermail archiver and then perform some other process once the message has been archived and its location is known.
Ext_Arch.py is a template for an external archiver that invokes pipermail to archive the message and then is able to call some other process once things like the archived message's URL and location in the file system are known.