Anselm Lingnau writes:

.... indexing big mail archives from Zope .... You can use the Zope product "LocalFS" (maybe also "ExternalFile") to wrap external files into Zope objects (without storing then in the ZODB).

Newer versions of "LocalFS" allow you the catalog its contents with ZCatalog. However, before you commit to ZCatalog, make some performance measures. ZCatalog tends to require large amounts of time and memory to build its indexes. Dieter