It should be XML, with a clearly defined format, to be stable and usable by other mail clients, too.
The [http://developer.mozilla.org/en/docs/Thunderbird_ISP_hooks old/current RDF file format ] seems good in structure, but needs to be cleaned up to remove the RDF bits, Mozilla specifics etc.. It would be processed as normal XML, e.g. using E4X in Thunderbird.
The email address (before @ or with domain) that the user entered can be used as placeholder in the config file, so the file is the same for all users (i.e. static).