I have a few gigabyte of old mail, mostly unsorted as nnml format (i.e. Gnus, with one file per mail). How can I turn that mail into something useful, i.e. something I can search and browse easily and fast (i.e. grep takes far to long). I have no need for a full mail client, just the archive management part of it.
You might find the Notmuch mail indexer library. It is designed so that it can be used as a backing store for some mail client, but can also be used as independent mail index.
It maintains a Xapian full text index of your mail, so search queries should be a lot more efficient than
greponce you have all the messages imported.