> As for searching the archive, there are solutions like Elastic Search,
> Solr, lucene. Can we use one of them to search directly through the maildir.
Not quickly. Many archives will have thousands of messages, some will have
hundreds of thousands. There's no reasonable way to avoid indexes when
you've got more than "dozens" of files. The stats alone will kill performance.
If you want a full-text solution, you're going to have to use a lot of
(typically as much as the compressed archives themselves, and for some