postgresql and other databases are a possibility. Directories
full of carefully-named files might be close enough in practice,
though.
Storing the data as entire blobs might be a use for XML, though it might be better to just export to that. On the other hand, if the raw data is stored in a directly browsable way...
use munpack, or some perl Mime parser, to split out attachments?
Is there some ss-like layer on top of tcl, for a
simple olc-like client?